I spent 10 minutes surfing around, and overall, I really like the layout, sparklines, and navigation.

It looks like you're using Highcharts for the main graphs, and if they support transitions out-of-the-box, that'd be nice to have that turned on. For example, when zooming out from 1M to 1Y, it's a little tough to see the recent 1 month changes in the context of 1 year. The axis transitions just makes it a bit more distracting, without the line chart transitions between date ranges.

Also, making the selected range saved to the URL to make the links shareable would be a good feature to add too.

I see that it's built in React, but I'm curious why clicking on an individual stock appears to "leave" the app, not giving it quite as smooth of a single page app experience? Not a huge deal, just curious.

Otherwise, this looks really great!