When loading the data in a ChartWrapper like that (using the "dataSourceUrl" parameter), you don't have access to the DataTable used by the chart, so you can't change the column headers. If you switch to using a Query object, then changing them is easy:
function drawVisualization () {
// use the #setQuery method if you want to write a select statement
query.send(function (response) {
var data = response.getDataTable();
// change column 0's label to "Dimension 1"
data.setColumnLabel(0, 'Dimension 1');