(using excel 2007)

Sorry if this has been answered, I have tried to find similar with no luck.

I have several days of data, measured hourly. I want to plot a line graph so
that the y axis shows concentration (easy enough) but the x axis shows days
while plotting in hours.

I have tried a number of options but I always end up with either with
several lines (instead of 1 line running throughout that I am after) or have
120 hours worth of points running along the bottom, it looks cluttered and
often unreadable (this however doesn't really matter). I would like to try
and segregate each 24hr period of those 120hours so it will read 'Day 1, day
2...' at the bottom, but still plot each hour.

Any help will be much appreciated