X and Y axes are crowded and need updating

[mroberge]

• Flow duration X axis: "Number of measurements that exceed this discharge" should be shortened to 'exceedance'; perhaps a full explanation of how flow duration curves work could be put into the help page.
• Flow duration Y axis should match the Y axis on the hydrograph.
• Dates on the hydrograph are too crowded on small screens. Use fewer ticks. Perhaps add more ticks for larger screens, but design for small screens first.
• Are all of the data from Germany and the UK given as discharge, or are some stage? This should be reflected in the hydrograph Y axis. Also, all units should be converted to metric.