Small Enhancements

I made a few small enhancements to the GUI.  

The graph shows two lines. The trap line tracks the temperature-pressure curve of the trap.  The tank line tracks the temperature-pressure curve of the tank.  These two lines now display in different colors.

The text for each of these lines now display in their corresponding color.

Each line now has an X on the current data point.  This helps with displaying the data if the line folds back on itself.  

A ring buffer was added to each line to track its data.  This is required due to changes in the GTK library.  The ring buffers hold each datapoint to be graphed for each line.  These ring buffers can be easily resized in code.  The current setting allows for the storage of 1000 datapoints per line.