Evolution graph on top of every report: better context for data analysis #8096

The goal of this issue is to design a new mechanism to let users view an Evolution graph on top of each Piwik report page.

Why it is important

Piwik aims to provide a great experience to all users, and help them gain insights from their data. When doing data analysis, a common requirement is to look at how the metrics change over time. Having the metric value (and relative % value) for the selected date is useful, but often it is not enough. Users need to see whether the metric is decreasing or increasing. By giving users a quick view into the historical data variations, we provide information with high value in context. Other tools such as Google Analytics also display such graph on top of every page.

Suggested requirements

• by default, we do not show the graph on top of each report
• a user can click a new button/icon or button+text, and when clicked the Evolution graph could be displayed on top, ie. the evolution graph can be expanded / collapsed by the user in any page
• Piwik remembers, for a given report, whether the user wants to see an evolution graph at the top or not
• the evolution graph will display the metric that makes most sense for the given report page where it is displayed. For example:
  • in the "Visitors" menu, for each submenu report, the graph will show the Unique visitors or Visits metric (depending on whether period is day or week or month, one of either will be used)
  • in the "Actions" menu, the metric displayed on the graph depends on which submenu eg.
  • Actions>Pages, Page titles, etc. -> graph shows Pageviews metric
  • Actions->Exit pages -> graph show Exits metric
  • Actions> Outlinks > graph shows Outlinks metric
  • etc.
• In Referrers > Websites, graph shows "Visits" and "Visits from websites"
• in Referrers > Campaign, graph shows "Visits" and "Visits from campaign"
• Note: Goal reports already have the evolution graph at the top

As usual, feedback is welcome!