Clear scatterplot size field default

[brooks-eco]

Love your work. Great plugin.

When using plotly scatterplot the size field is automatically populated with the first column of the data table which may not be numeric creating an error.

The user can manually clear the size field and set the size using the slider tool. This works provided the tab is not refreshed. If the user edits a different tab and comes back the size resets to the field name again and the error is shown again because the db field is not numeric. I found while editing scatterplots I was flipping back and forth trying different options and the size continually was resetting.

The user experience would be improved if the size dropdown had no default and did not refresh (or at least remembered that it was empty and did not try to repopulate itself). size should stay set to the chosen value (slider) or field(if selected). I think this might just require the size dropdown to include a "blank" entry as the first entry of the dropdown.