Trip - time spent plot

I am trying to use the "trip" package to generate a map of time spent from 
a SpatialPointsDataFrame object using the tripGrid function and am having 
difficulty generating the final plot. The documentation seems to say that 
tripGrid generates a grid, carries out an interpolation, bins these 
interpolated points, and then counts the points allocated to each grid but 
it is not clear how to generate the final plot. Plotting the output of 
tripGrid just plots the grid itself. Does anyone have experience of using 
this package and advise me on how to generate the final time-spent plot?