Skip drawing the line for negative values in log plots

[certik]

No description provided.

[jkriege2]

Hi and thanks for the contribution ... i have two requests:

1. Please post a Screenshot of the old vs. The fixed plots that illustrate the problem and solution
2. Why do you use a vector of uniqe_ptr instead of a simple vector of polygons?
   Best, jan

[certik]

Regarding 1., here are two screenshots of the current behavior. It's the same graph moved using the "hand" tool:

as you can see, the graph changed it's shape simply by moving it. That is not optimal.

In this PR, the graph looks like this:

and it is invariant when you move it around with the "hand" tool. As it should be. Matplotlib has the same behavior as this PR.

[certik]

Regarding 2., that was fixed in ae722e8.

[jkriege2]

OK!

great addition!

Thanks,
JAN