code: public static PlotCanvas plot(double[][] data, int xbins, int ybins) {
          return plot(data, xbins, ybins, null);
      }
docs: Create a plot canvas with the histogram plot of given data.
  @param data a sample set.
  @param xbins the number of bins on x-axis.
  @param ybins the number of bins on y-axis.