Add axis to numerical table columns

[alexsb]

For numerical values, add a permanently visible axis, like this one (but on top):

[anniecherk]

Implementation sketch:

• Add the axis below label
• Only display axis if there is "enough" room: otherwise with many attributes this becomes illegible
• Label low / mid / high points
  

Issues:

• making sure the edge values are legible if the neighboring column also has a labeled axis
• allow the user to specify a "reference value" in the panel and either:
  a) display it normally along with the other values
  b) display it instead of the mid value
  c) display it in bold along with the other values

[alexsb]

Looks good.

The reference value could also be specified in the panel, but it could also be in both.
Re labeling - yes, there needs to be enough spacing.

[alexsb]

Implemented!