Add the functionality to add a line (horizontal, vertical) to the plots

[TatianaBurek]

Describe the Enhancement

METviewer has a special field <plot_cmd> to add an extra feature to the plot using Rscript functions. Mostly this field is used to add vertical or horizontal line. For example, y=1 line.:
<plot_cmd>abline(h=1, col="blue")</plot_cmd>

This feature is not secure and should not be used as is. But it is important that we continue to support the addition of user specific lines.
We will add this functionality in Python scripts but need the supporting changes in METviewer:

• change UI in METviewer (instead of providing R command let the user to specify the line using new fields).
• update XML with new tags

Corresponding issue in METplotpy:
dtcenter/METplotpy#140

Time Estimate

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Issues should represent approximately 1 to 3 days of work.

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Funding Source

2702691

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Define Related Issue(s)

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• METplus, MET, METdatadb, METviewer, METexpress, METcalcpy, METplotpy

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