An irritating quirk with my scatterplot graph

I’m essentially done with the scatterplot graph - at least the FCC testing script tells me so - but I’m stuck with a very irritating quirk. When I mouseover a point on the graph I am supposed to be able to see a tooltip with some information. This happens about half the time. The other half, the cursor turns into a pointer and the tooltip does not appear. The issue is the same on Chrome, Firefox, and Safari, so I’m pretty sure the issue isn’t the browser.

Here’s a link to my project’s GitHub Page, and here’s a link to my project’s repository (Sorry, I’m not a fan of Codepen).

Any suggestions would be greatly appreciated.