Typically, we compare two colors when checking contrast for accessibility, like the two below. 

And the reason why we might do that is make sure that we can compare two data elements, like the donut chart below. 

While we need to distinguish the two colors for readability and accessibility, we also need to consider...there's a third color.   It wasn't until I had an experience of testing two colors for accessibility that I thought, "wait, these colors are going to need to be seen against the background too!"  Thus, our third color...our background color.  

What I've learned is to test the color closest to the background to ensure that the data elements are accessible compared to each other and the background.   In this example and from the image below, we can see that there is insufficient contrast between the white background and color/data point 2.  

So we need to make an adjustment (maybe that's change the color, outline the color in a darker color, or add markers for the data points---it just depends on what will help the audience read the visualization). 

And that's what I learned...check your colors against each other AND the background.

As I was completing Chartability for one of my vizzes, I found myself wondering,
"How do I check for keyboard navigation?"  
"What's a tab stop?"

That's when I reached out to my go-to resource, who seems always willing to help, Frank Elavsky. 

Here's what I took away from that discussion:

1. Launch your page that includes your dashboard
2. Click in to the top left corner.
3. Tab through, noting where the focus is.
   1. Was anything missed?
   2. Was everything in the order in which you intended it to be?
   3. Check it against Chartability

Here are two quick demos of what it looks like on two of my vizzes that were completed in 2020 and 2021. You can see that the border around the focus area is hard to discern from the colors used in the viz.  I'll be looking at seeing if/how I can change the border.  

Try it for yourself and see what happens!

I also put together this speed tip for the #a11ytug which covers what keyboard navigation is, why it's important, how to navigate/test with a keyboard only, and the heuristics from Chartability.  

Check it out & let me know if you have any questions!