A shameless copy of the Matplotlib plotting interface, using Tim Molter's XChart library to do the heavy lifting.

I've really written this for my own benefit, because I like the simple interface of Matplotlib. To show how concise it can be, here's how to display a simple XY plot in 3 lines of code (in this example the single array of doubles is used for both the x and y data):

Plotter plt = new Plotter();
plt.plot(new double[] {1, 2, 3});
plt.show();

This being Java rather than Python, I've provided a constructor that allows you to define the title, the x-axis label and the y-axis label:

Plotter plt = new Plotter("My Chart Title", "My X-Axis Label", "My Y-Axis Label");
...

But if you want to do it the Python way, you can do that too:

Plotter plt = new Plotter();
plt.title("My Chart Title");
plt.xlabel("My X-Axis Label");
plt.ylabel("My Y-Axis Label");
...

Each call to plot creates a new series on the chart, with a default legend of 'Series 1', 'Series 2', etc. If you want to specify the legend yourself, just pass that as a third parameter in the call to plot.

Plotter plt = new Plotter();
plt.plot(new double[] {1, 2, 3}, new double[] {1, 2, 3}, "My Series Legend");
plt.show();

The colour, line-style and marker-style of the plot will default to something sensible, and each time you call plot you'll get a new plot style by default. But if you want to set the style yourself, you can define it using the standard Matlab format string, where the first character defines the colour, the second character defines the marker style, and the remaining characters define the line-style (the marker and line characters are optional). For example, the following code will create a red plot with upright triangular markers and a solid line:

Plotter plt = new Plotter();
plt.plot(new double[] {1, 2, 3}, new double[] {1, 2, 3}, "My Series Legend", "r^-");
plt.show();

If you don't provide a marker character, no marker will be printed.

If you don't provide a line character, no line will be printed.

The chart can be saved to an image file by calling savefig. The supported file formats are bmp, jpg, png and gif. Note that you need to call show before you call savefig because the image file is generated from the displayed chart.

Plotter plt = new Plotter();
plt.plot(new double[] {1, 2, 3}, new double[] {1, 2, 3}, "My Series Legend", "r^-");
plt.show();
plt.savefig("MySampleChart.jpg");

int size = 30;
double[] xData1 = new double[size];
double[] xData2 = new double[size];
double[] yData1 = new double[size];
double[] yData2 = new double[size]; 
for (int i = 0; i < size; i++) {
 double radians = (Math.PI / (size / 2) * i);
 xData1[i] = xData2[i] = i - size / 2;
 yData1[i] = -.000001 * Math.sin(radians);
 yData2[i] = -.000001 * Math.cos(radians);
}
   
Plotter plt = new Plotter("Example sin and cos plots", "x", "f(x)");
plt.plot(xData1, yData1, "sin(x)");
plt.plot(xData2, yData2, "cos(x)");
plt.show();

To create multiple charts on a single figure, use subplots(rows, cols), to set up the matrix of charts, then call subplot(index) to position the plotter on the chart you want to add data to. Note that indexing starts at 1, and the matrix is traversed from left to right and top to bottom.

Those familiar will matlab/matplotlib will notice that I've diverged from the interface here. But I find the matlab api for subplots overly complex, so I've decided not to copy it slavishly.

Plotter plt = new Plotter();
plt.subplots(2, 2);
        
for (int i = 1; i <= 4; i++) {
    double[] x = getConsecutiveValues(200);
    double[] y1 = getRandomWalk(200);
    double[] y2 = getRandomWalk(200);
    
    plt.subplot(i);
    plt.title("Title " + i).xlabel("X Label " + i).ylabel("YLabel " + i);
    plt.plot(x, y1, "Series A" + i, "b-");
    plt.plot(x, y2, "Series B" + i, "r-");
}

plt.show();