Anomalous legend for multiline graph with non-date x-axis values

[aashaka]

For multiline graph, when x-axis has numeric values, the legend has an extra "key" and "multiline_y_accessor" tag added to it (Example).

Upon changing the x-accessor to a date format for the same data, the legend comes out to be just fine (Example).

I found the following functions to be important for this issue: mg_process_multiple_y_accessors and mg_format_data_for_mouseover. But it may need more looking into.