Creating appropriate time axis for data

Hello,

I am dealing with data stored in a database as a 'time' object.  I
export the data from the database to a text file and utilize the
'time_to_sec()'

function of the database to convert the human readable
time (HH:MM:SS) to seconds so that I can use R to do analysis and
create charts of the data.  I do not need or use days or dates in the
data, and the data range is from 07:00:00 to 18:00:00 or 25200-64800
seconds.  However when I visualize the data in charts I do not want a
scale that runs from 25200-64800 seconds, but rather in the HH:MM:SS
format.  Is there a relatively straight-forward and easy to use way to
do this?

Thanks,
Tony