Lines are invisible (out of canvas) when smoothing is on

[ultmaster]

Hi all. Thanks for building this great toolkit.

Recently I found that turning on smoothing is a good choice. It makes the chart look nicer and sometimes more friendly. But I notice that some of the lines are invisible (overflow out of canvas) when smoothing is on.

The following snapshot is taken directly from the demo github.io page:

It looks a more serious problem in my case:

I googled. Stackoverflow says that there is no such thing as overflow: visible for canvas. Canvas needs to be enlarged to make sure everything is drawn inside.

Any workarounds on this? Thanks.