Chronology tab: use stepped graph instead of linear interpolation

[tyrasd]

When a tag or key is updated infrequently, the corresponding chronology graph – which currently uses a linear interpolation to fill gaps in the data – may give a false impression of the real development of the counts. Here's an example:

It would be better to use a stepped line as in taghistory:

[joto]

You are right, the taghistory graph is more correct. This should be fixed.