optional vector of colors as long as the maximum number of revisits to color code trajectory points
alpha
optional alpha value for color transparency between 0 and 1
legendPos
a vector of length 2 with the x and y coordinate of the center of the legend in user coordinates
Author
Chloe Bracis <cbracis@uw.edu>
Details
This method allows the user to visually represent the number of revisitations by location. The size
of the circle of radius R can be added to the plot with drawCircle.