Apply column stratification of first categorical column correctly

[thinkh]

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• Web browser version and OS:
• Environment (local or deployed):

Steps to reproduce

1. Add a matrix (e.g. Number1Hits)
2. Add a categorical column for this matrix (e.g. continent)
3. Split matrix by min/mean/max (type doesn't matter)

Observed behavior

• The matrix is not splitted into 3 vectors as expected
• The user first has to click the stratify button of the categorical column and then split

Expected behavior

• The stratification should be applied automatically when adding the first categorical column to a matrix

[mstreit]

The TCGA stratification are already available in this branch: https://github.com/Caleydo/taggle/tree/sgratzl/stratification However, as every gene can be a stratification we run into a scalability problem for the select2 search.

[bikramkawan]