code: public static PlotCanvas plot(double[][] data, char legend, Color color) {
          return plot(null, data, legend, color);
      }
docs: Create a plot canvas with the scatter plot of given data with specific legend and color.
  @param data a n-by-2 or n-by-3 matrix that describes coordinates of points.
  @param color the color used to draw points.
  @param legend the legend used to draw points.
  <ul>
  <li> . : dot
  <li> + : +
  <li> - : -
  <li> | : |
  <li> * : star
  <li> x : x
  <li> o : circle
  <li> O : large circle
  <li> @ : solid circle
  <li> # : large solid circle
  <li> s : square
  <li> S : large square
  <li> q : solid square
  <li> Q : large solid square
  <li> others : dot
  </ul>