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Documentation

Installation

<link rel="stylesheet" href="dist/uPlot.min.css">
<script src="dist/uPlot.iife.min.js"></script>

Data Format

let data = [
  [1546300800, 1546387200],    // x-values (timestamps)
  [        35,         71],    // y-values (series 1)
  [        90,         15],    // y-values (series 2)
];

uPlot expects a columnar data format as shown above.

  • x-values must be numbers, unique, and in ascending order.
  • y-values must be numbers (or nulls for missing data).
  • x-values and y-values arrays must be of equal lengths >= 2.

By default, x-values are assumed to be unix timestamps (seconds since 1970-01-01 00:00:00) but can be treated as plain numbers via scales.x.time = false. JavaScript uses millisecond-precision timestamps, but this precision is rarely necessary on calendar-aware time: true scales/plots, which honor DST, timezones, leap years, etc. For sub-second periods, it's recommended to set time: false and simply use ms offsets from 0. If you truly need calendar-aware ms level precision, simply provide the timestamps as floats, e.g. 1575354886.419. More info....

This format has implications that can make uPlot an awkward choice for multi-series datasets which cannot be easily aligned along their x-values. If one series is data-dense and the other is sparse, then the latter will need to be filled in with mostly null y-values. If each series has data at arbitrary x-values, then the x-values array must be augmented with all x-values, and all y-values arrays must be augmented with nulls, potentially leading to exponential growth in dataset size, and a structure consisting of mostly nulls.

This does not mean that all series must have identical x-values - just that they are alignable. For instance, it is possible to plot series that express different time periods, because the data is equally spaced.

Before choosing uPlot, ensure your data can conform to these requirements.

Basics

let opts = {
  title: "My Chart",
  id: "chart1",
  class: "my-chart",
  width: 800,
  height: 600,
  series: [
    {},
    {
      // initial toggled state (optional)
      show: true,

      spanGaps: false,

      // in-legend display
      label: "RAM",
      value: (self, rawValue) => "$" + rawValue.toFixed(2),

      // series style
      stroke: "red",
      width: 1,
      fill: "rgba(255, 0, 0, 0.3)",
      dash: [10, 5],
    }
  ],
};

let uplot = new uPlot(opts, data, document.body);
  • id and class are optional HTML attributes to set on the chart's container <div> (uplot.root).
  • width and height are required dimensions in plotting area, axes & ticks, but excluding title or legend dimensions (which can be variable based on user CSS).
  • spanGaps can be set to true to connect null data points.
  • For a series to be rendered, it must be specified in the opts; simply having it in the data is insufficient.
  • All series' options are optional; label will default to "Value" and stroke will default to "black".
  • width is the series' line width in CSS pixels.
  • stroke, width, fill, and dash map directly to Canvas API's ctx.strokeStyle, ctx.lineWidth, ctx.fillStyle, and ctx.setLineDash.

High/Low Bands

High/Low bands are defined by two adjacent data series in low,high order and matching opts with series.band = true.

const opts = {
  series: [
    {},
    {
      label: "Low",
      fill: "rgba(0, 255, 0, .2)",
      band: true,

    },
    {
      label: "High",
      fill: "rgba(0, 255, 0, .2)",
      band: true,
    },
  ],
};

Series, Scales, Axes, Grid

uPlot's API strives for brevity, uniformity and logical consistency. Understanding the roles and processing order of data, series, scales, and axes will help with the remaining topics. The high-level rendering flow is this:

  1. data is the first input into the system.
  2. series holds the config of each dataset, such as visibility, styling, labels & value display in the legend, and the scale key along which they should be drawn. Implicit scale keys are x for the data[0] series and y for data[1..N].
  3. scales reflect the min/max ranges visible within the view. All view range adjustments such as zooming and pagination are done here. If not explicitly set via opts, scales are automatically initialized using the series config and auto-ranged using the provided data.
  4. axes render the ticks, values, labels and grid along their scale. Tick & grid spacing, value granularity & formatting, timezone & DST handling is done here.

You may have noticed in the previous examples that series and axes arrays begin with {}. This represents options/overrides for the x series and axis. They are required due to the way uPlot sets defaults:

  • data[0], series[0] and axes[0] represent & inherit x defaults, e.g:

    • "x" scale w/ auto: false
    • temporal
    • hz orientation, bottom position
    • larger minimum tick spacing

  • data[1..N], series[1..N] and axes[1..N] represent & inherit y defaults, e.g:

    • "y" scale w/ auto: true
    • numeric
    • vt orientation, left position
    • smaller minimum tick spacing

While somewhat unusual, keeping x & y opts in flat arrays [rather than splitting them] serves several purposes:

  • API & structural uniformity. e.g. series[i] maps to data[i]
  • Hooks receive an unambiguous i into the arrays without needing further context
  • Internals don't need added complexity to conceal the fact that everything is merged & DRY

More thoughts in #76 & #77.

Multiple Scales & Axes

Series with differing units can be plotted along additional scales and display corresponding y-axes.

  1. Use the same series.scale key.
  2. Optionally, specify an additional axis with the scale key.
let opts = {
  series: [
    {},
    {
      label: "CPU",
      stroke: "red",
      scale: "%",
      value: (self, rawValue) => rawValue.toFixed(1) + "%",
    }
    {
      label: "RAM",
      stroke: "blue",
      scale: "%",
      value: (self, rawValue) => rawValue.toFixed(1) + "%",
    },
    {
      label: "TCP",
      stroke: "green",
      scale: "mb",
      value: (self, rawValue) => rawValue.toFixed(2) + "MB",
    },
  ],
  axes: [
    {},
    {
      scale: "%",
      values: (self, ticks) => ticks.map(rawValue => rawValue.toFixed(1) + "%"),
    },
    {
      scale: "mb",
      values: (self, ticks) => ticks.map(rawValue => rawValue.toFixed(2) + "MB"),
      side: 1,
      grid: {show: false},
    },
  ],
};
  • side is the where to place the axis (0: top, 1: right, 2: bottom, 3: left).

Axes for Alternate Units

Sometimes it's useful to provide an additional axis to display alternate units, e.g. °F / °C. This is done using dependent scales.

let opts = {
  series: [
    {},
    {
      label: "Temp",
      stroke: "red",
      scale: "F",
    },
  ],
  axes: [
    {},
    {
      scale: "F",
      values: (self, ticks) => ticks.map(rawValue => rawValue + "° F"),
    },
    {
      scale: "C",
      values: (self, ticks) => ticks.map(rawValue => rawValue + "° C"),
      side: 1,
      grid: {show: false},
    }
  ],
  scales: {
    "C": {
      from: "F",
      range: (self, fromMin, fromMax) => [
        (fromMin - 32) * 5/9,
        (fromMax - 32) * 5/9,
      ],
    }
  },
  • from specifies the scale on which this one depends.
  • range converts from's min/max into this one's min/max.

Scale Opts

If a scale does not need auto-ranging from the visible data, you can provide static min/max values. This is also a performance optimization, since the data does not need to be scanned on every view change.

let opts = {
  scales: {
    "%": {
      auto: false,
      range: [0, 100],
    }
  },
}

The default x scale is temporal, but can be switched to plain numbers. This can be used to plot functions.

let opts = {
  scales: {
    "x": {
      time: false,
    }
  },
}

A scale's default distribution is linear distr: 1, but can be switched to indexed/evenly-spaced. This is useful when you'd like to squash periods with no data, such as weekends. Keep in mind that this will prevent logical temporal tick baselines such as start of day or start of month.

let opts = {
  scales: {
    "x": {
      distr: 2,
    }
  },
}

Axis & Grid Opts

Most options are self-explanatory:

let opts = {
  axes: [
    {},
    {
      show: true,
      label: "Population",
      labelSize: 30,
      labelFont: "bold 12px Arial",
      font: "12px Arial",
      gap: 5,
      size: 50,
      stroke: "red",
      grid: {
        show: true,
        stroke: "#eee",
        width: 2,
        dash: [],
      },
      ticks: {
        show: true,
        stroke: "#eee",
        width: 2,
        dash: [],
        size: 10,
      }
    }
  ]
}
  • size & labelSize represent the perpendicular dimensions assigned to values and labels DOM elements, respectively. In the above example, the full width of this y-axis would be 30 + 50; for an x-axis, it would be its height.
  • gap is the space between axis ticks and values.

Customizing the tick/grid spacing, value formatting and granularity is somewhat more involved:

let opts = {
  axes: [
    {
      space: 40,
      incrs: [
         // minute divisors (# of secs)
         1,
         5,
         10,
         15,
         30,
         // hour divisors
         60,
         60 * 5,
         60 * 10,
         60 * 15,
         60 * 30,
         // day divisors
         3600,
      // ...
      ],
      // [0]:   minimum num secs in found axis split (tick incr)
      // [1]:   default tick format
      // [2-7]: rollover tick formats
      // [8]:   mode: 0: replace [1] -> [2-7], 1: concat [1] + [2-7]
      values: [
      // tick incr          default           year                             month    day                        hour     min                sec       mode
        [3600 * 24 * 365,   "{YYYY}",         null,                            null,    null,                      null,    null,              null,        1],
        [3600 * 24 * 28,    "{MMM}",          "\n{YYYY}",                      null,    null,                      null,    null,              null,        1],
        [3600 * 24,         "{M}/{D}",        "\n{YYYY}",                      null,    null,                      null,    null,              null,        1],
        [3600,              "{h}{aa}",        "\n{M}/{D}/{YY}",                null,    "\n{M}/{D}",               null,    null,              null,        1],
        [60,                "{h}:{mm}{aa}",   "\n{M}/{D}/{YY}",                null,    "\n{M}/{D}",               null,    null,              null,        1],
        [1,                 ":{ss}",          "\n{M}/{D}/{YY} {h}:{mm}{aa}",   null,    "\n{M}/{D} {h}:{mm}{aa}",  null,    "\n{h}:{mm}{aa}",  null,        1],
        [0.001,             ":{ss}.{fff}",    "\n{M}/{D}/{YY} {h}:{mm}{aa}",   null,    "\n{M}/{D} {h}:{mm}{aa}",  null,    "\n{h}:{mm}{aa}",  null,        1],
      ],
  //  splits:
    }
  ],
}
  • space is the minimum space between adjacent ticks; a smaller number will result in smaller selected divisors. can also be a function of the form (self, axisIdx, scaleMin, scaleMax, dim) => space where dim is the dimension of the plot along the axis in CSS pixels.
  • incrs are divisors available for segmenting the axis to produce ticks. can also be a function of the form (self) => divisors.
  • values can be:
    • a function with the form (self, ticks, space) => values where ticks is an array of raw values along the axis' scale, space is the determined tick spacing in CSS pixels and values is an array of formatted tick labels.
    • array of tick formatters with breakpoints.