Support strings as X values in chart

[wander]

I'd like to write arbitrary strings as X values in a #chart, e.g.:

#chart;lines;points;legend;legend-position:se
X |Y Validation ratio (\%) |
May | 4.6 |
    | 3.6 |
    | 3.7 |
Jun | 4.0 |
    | 4.5 |
    | 5.3 |
    | 4.6 |
    | 5.0 |
Jul | 4.7 |
    | 4.5 |
    | 5.2 |
    | 5.7 |
Aug | 5.0 |
    | 4.9 |
    | 4.8 |
#

Non-empty strings trigger a warning (Malformed value in chart data: May) and empty strings trigger a runtime error (panic: runtime error: index out of range).