Average Protein Level graph is not bounded at min and max y values

[phet-steele]

The average protein level can go both above and below the graph. The values should probably stay clamped so this does not occur.

Just let one cell run over time (with optimal protein synthesis conditions) and eventually this will happen:

Here, also let one cell run. Make it lose all of its protein and the line will dip below the graph (for demonstration, I then kicked the cell back into high gear to show the line had indeed gone below the graph).

In both of these situations, the line should travel flat along the x axis whenever it has exceed the min/max y values.

Seen on macOS 10.12.3 Chrome and by @BryceAG on Win 7. For phetsims/tasks/issues/776.

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[phet-steele]

In both of these situations, the line should travel flat along the x axis whenever it has exceed the min/max y values.

@aadish, I actually like what the Java does better. When the graph exceeds the max y, the line is clipped:

We should do that, too. However, we should keep the behaviour I suggested for the min y values (a flat line)

[phet-steele]

I love it! Thanks @aadish.