Each data file has a 'Plot' link next to it. Following the link will open another window which will display a plot of a selected parameter against date. You can change the plot parameter using the form at the bottom of the graph. Details of the parameters can be found by following the 'File format information' link. The plotting works best for files which contain a single daily measurement.
Some file types contain data taken more than once per day.
Plotting these files will produce a vertical column of values for each day. These plots are still valid, but are not as easy to interpret.