Hexbin: default height for lowest value.

**Is your feature request related to a problem? Please describe.**
It feels misleading when using the hexbin height, that the lowest value does not get a height.
When I use _Point count_ as the height's source, 1 is flat, 2 is 1 height unit, 3 is 2 height units.
But to a spectator, 2 looks much higher than 1.

**Describe the solution you'd like**
To make the value _0_ flat, and not the lowest value.

**Describe alternatives you've considered**
To calculate the number of points per coordinate myself, add a row with value 0, and then plot that.

**Additional context**
The flat ones are value 1, the middle value 2 and the highest is value 3.
It really doesn't feel that way looking at it with fresh eyes.