Implement zoom-in / zoom-out feature

[Ruke256]

Having to see the entire chart without the trouble of scrolling could be a useful feature, especially if the chart is pretty big.

[amanharwara]

Have implemented this just now. It's probably not perfect, but it should work at least decently. Also right now it is ephemeral in the sense that it is not stored in the actual chart and gets reset on refresh. That's because I'm working on changing how the chart data is structured and stored for better performance and fixing some bugs, and I don't want to add any new things to it for now.