Add HTTPS support to gad-data-model.php

  function create_google_chart_url($width, $height)
  {
    $chart_url = 'http://chart.apis.google.com/chart?chs=';
    // Change the chart source URL for secure (HTTPS) pages
    if( isset( $_SERVER['HTTPS'] ) && 'on' == $_SERVER['HTTPS'] ) {
        $chart_url = 'https://chart.googleapis.com/chart?chs=';
    }
    return $chart_url . $width . "x" . $height . "&chf=bg,s,FFFFFF00&cht=lc&chco=0077CC&chd=t:" . $this->chart_values . "&chds=" . ($this->minvalue - 20). "," . ($this->maxvalue + 20) . "&chxt=x,y&chxl=0:" . $this->x_axis_labels . "&chxr=1," . $this->minvalue . "," . $this->maxvalue . "&chxp=0," . $this->y_axis_labels . "&chm=V,707070,0," . $this->first_monday_index . ":" . $this->total_count . ":7,1|o,0077CC,0,-1.0,6";
  }