JavaScript Graphing Calculator
Increase the interval but shrink the view

Because you're plotting points you can't just increase the number of points you have, as you know, that will take up much more processing power. To mitigate this processing power issue, you need to shrink the view of your graph to display a smaller area. This has the effect of zooming in.

If you were using a more advanced way to draw the functions (i.e. estimating the area between points and drawing curves) you would most likely not have this issue, but using only points and a for loop limits you.

Another problem I barely noticed, you're overdrawing. It would seem that you're plotting past the borders of your canvas/graph. This causes unnecessary drawing that slows down everything. You should be basing the size of the area you're drawing on the width and height of the canvas.