Make middle strip in geomCrossbar() optional

[AndreiKingsley]

This is a fairly common usecase , i.e. the user needs the bars themselves, without the middle bar. That is, I'd like to propose the following behavior: if y is not assigned, the bar is not drawn (as it is currently when passing a list of nulls).
Actual:

Expected:

We did already the same for boxplot: #143

[alshan]

Hi, try using fatten = 0

[AndreiKingsley]

Ok, but let's get rid of middle bars on 0, why not?

[alshan]

0 is a legit value.

[AndreiKingsley]

I mean if y is not assigned, don't draw a middle bar at all instead od drawn it at 0

[alshan]

Got it.

[alshan]

Fixed by commit: JetBrains/lets-plot@8a554dc