Extend/line graph

Two new features:

- Chart always shows only display values. If it is blank, it will not show any values. There is a switch to set display values equal to values (if they are the same, and to avoid editor having to type it twice. If we like this setup, I can apply it to other charts as well.

- Added ability to customize y-scale (i.e. start from 0). By default, the y-scale is based on min/max values.