plots: standardise across DVC/Studio/VS Code (#142)

* update templates with new anchors

* prototype filling optional anchors

* move update datapoints to top level

* add color into anchor definitions

* add get_revs method to renderer

* add anchors to fix confusion templates

* split x and y labels out so they can be truncated by vs code

* remove erroneous comment

* fix issue with confusion matrix

* fix string issues

* add title to split anchors

* add zoom and pan anchor

* add tooltip anchor for Studio

* rename anchor_revs to revs_with_datapoints

* add empty sort property to order facet by rev in datapoints

* add width and height anchors

* add sort to y offset in horizontal bar templates

* move horizontal bar plots from row to column

* remove row_height and column_width anchors

* use field separator for pivot field

* fix linear plots with varied filename field

* extend optional anchor tests

* ensure all 4 variations have test cases

* hoist anchors_y_definition in tests

* add separate color test