Improve the "Comparison per year" report/widget

[sebastfr]

Reported by many.

The different graphs are not on the same scale. Mostly because each graph is unaware of the other graphs (the data is fetched in different Ajax calls) so it's hard to compute and overall maximum y-value over the different graphs.

Perhaps this should be plotted on one single graph, with multiple line with different colours.

[mpbraendle]

In addition, the menu buttons "Filter items", "Dates", and "Available Reports" are missing. There is no way to return back to another (or previous report) than using the Back button of the browser.

[jesusbagpuss]

@mpbraendle If you change your config in z_irstats2.pl (https://github.com/eprints/irstats2/blob/master/cfg/cfg.d/z_irstats2.pl#L453-#L459):

compare_years => {
    items => [
        { plugin => 'SetDesc' },
        { plugin => 'Compare' },
    ],
    category => 'general',
},

to

compare_years => {
    items => [
        { plugin => 'ReportHeader' },
#        { plugin => 'SetDesc' }, # removed - this plugin doesn't seem to exist!?
        { plugin => 'Compare' },
    ],
    category => 'general',
},

Then the normal report selection header should appear.