State that histogram bins must have uniform width. Closes clauswilke#94.

visualizing_distributions_I.Rmd

@@ -39,7 +39,7 @@ knitr::kable(
 
 ```
 
-We can visualize this table by drawing filled rectangles whose heights correspond to the counts and whose widths correspond to the width of the age bins (Figure \@ref(fig:titanic-ages-hist1)). Such a visualization is called a histogram.
+We can visualize this table by drawing filled rectangles whose heights correspond to the counts and whose widths correspond to the width of the age bins (Figure \@ref(fig:titanic-ages-hist1)). Such a visualization is called a histogram. (Note that all bins must have the same width for the visualization to be a valid histogram.)
 
 (ref:titanic-ages-hist1) Histogram of the ages of Titanic passengers.