Center line missing on interaction potential chart

[jbphet]

The current interaction potential chart on the "Phase Changes" screen is missing the horizontal center line. It looks like this:

The live version (v 1.1.10) looks like this:

[jbphet]

After doing some forensics, it looks like this was removed in an attempt to fix the problem that the center axis line goes over the potential (red) line in phetsims/atomic-interactions#75.

Specifically, here is what I think is the problematic commit: bc510f7

[jbphet]

I've made some changes, here's what it looks like now (using water as the substance, which makes the line more apparent):

[jbphet]

@SaurabhTotey - please review. It looks like you made the commit that removed the line, and I wanted to make sure that this wasn't intentional. If all looks good, please close.

[SaurabhTotey]

Oh, I completely missed this side-effect! It wasn't my intention to remove the line for the interaction potential chart, but it looks good now. Closing.