Chart UI accessibility #2217

Some accessibility-related things I noticed with the colors + text applied

• There are some instances where white text is used on green bars - for example fonts chapter has a graph where the % is shown as white on the green bar (failing text color contrast) vs accessibility where it's black

• Similar issues with white text on red bars - e.g. media chapter (failing text contrast)

• "heatmap" on the web font page is using a red-green scale, which isn't a color blind friendly option. Color brewer has a list of colour-blind friendly palettes we can pick for a diverging scale, including red-blue

• a lot of graphs have colors placed side by side which may not have sufficient contrast between them. If we add white dividers between the slices/bars/etc it should make it easier to see. E.g. e-commerce chart today. WCAG has some sample techniques on their site.

• generally wondering how the categorial colors are selected, and if we want to use one of color brewer's suggested ones