How to make a histogram widget

Depending on what you're trying to count, you might not need to have anything in the Y-Axis/Category at all. What gabrielladawn​ was suggesting is manually creating multiple separate formulas under the Values section, one for each range/bucket you want to show as a bar.

Here's something similar I did in my system, using a Column Chart instead of a Bar Chart:

Here's what the widget setup looks like:

Each one of those Values is a formula I created. Here's what the first one looks like (the rest are pretty similar, just swapping out numbers where appropriate):

The only thing that stinks about this method is the manually creation of each bucket. If you want a LOT of buckets then that needs a lot of formulas, and the ordering of them is also entirely manual. I'm guessing that's what the Histogram plugin is supposed to do, it probably just automates this process so you can dynamically increase or decrease the number of bars shown.