adding support for embedded functions inside the google visualization

I needed a new function inside the draw_chart() function to support custom tooltips for the TreeMap chart. Added a new add_function method along with additional spec for testing. Please advise?

Ex: @chart.add_function("   function showTooltip(row, size, value) {
    return '<div style=\"background:#99dcff; padding:10px; border-style:solid; font-size: small;\">' +
           '<span style=\"font-family:Times;\"><b>' + data_table.getValue(row, 1) + '<\b></span><br></div>';  }")
Should be https://developers.google.com/chart/interactive/docs/gallery/calendar instead.