Scatterplot: allow user to pick both the x and y axes

[GregoryTravis]

Currently you can pick the y axes but not the x axis.

[marthasharkey]

• similar component to the label picker
• on change recall process_to_json_text
• update process_to_json_text to take an optional param of selected column for x
• update the get_x_value_type to identify which column should be used as X