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* 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

* Fix restoration of ad-hoc variable value issue

* Escape delimiters in ad-hoc variable URL

* Remove unnecessary newline

* Add @ngInject to HeatmapCtrl constructor, issue #8013

* Remove unused loadPluginCss

This reverts commit 554f972a.

fix: sqlite3 and concurrent write requests caused database locked error some times, depending on how many concurrent requests and speed of disk drive. Fixed by reducign the number of concurrent db connections to 1 for sqlite3, fixes #7992

Merge branch 'heatmap-panel' of https://github.com/alexanderzobnin/grafana into alexanderzobnin-heatmap-panel