* graph: initial histogram support #600

* graph histogram mode: add Bars number option

* graph histogram mode: fix X axis ticks calculation

* graph histogram mode: change bar style (align and width)

* refactor(graph): move histogram functions into separate module

* graph histogram mode: rename series to "count"

* graph histogram mode: fix errors if no data

* refactor(graph and heatmap): move shared code into app/core

* graph: add tests for histogram mode