GChartPhp - using HTML "title" (hovertip) on the X axis labels

Question

I would like to have as many labels as possible across the bottom X-axis, which means that the text has to be short. For instance, on a date range, I just put a day number, but I would like the user to be able to er the cursor over a label and see the calendar date comprehending to day number, which is just 1 2 3 4 ....

So, I thought I could just use

  $_SESSION['chart']['date_labels'][] = 
             '<a title="calendar date">' . $i . '</a>';

but that doesn't seem to work.

Can anyone help?