Refs #8678. Fixed and refactored addCurves method.

Y column are now added to the beginning of the list of drawable
columns so that they are drawn first and can be later found to attach
error and label columns to.

Switch statement is used now instead of a bulk of ifs.
Refs #8678. Fixed colIndex method.

It was assuming that column names shouldn't have underscores in them,
which was not always true. A bit cleverer approach is now used.
Merge remote-tracking branch 'origin/bugfix/8678_table_plotting'