I believe the following 4 tags should be consolidated to google-visualization:
google-charts and google-charts-api questions are usually actually asking about google-visualization. Since there are very few users with enough reputation to identify the differences between the two and retag properly, the tags are currently a giant mess and aren't getting any better as time goes on.
All four of these topics have incredibly similar types of questions in them, and there are a lot of cross-tags. There are very few answerers who frequent one of these tags, but not the other three, but there are many people asking questions in one tag who may not be aware of the others. None of these tags get much traffic in general, meaning it is hard for the regulars to actually manage the tags due to insufficient reputation.
google-geochart is a subset of google-visualization, but there is no tag for 'google-barchart' or 'google-histogram' which are equivalent (and further would need to be distinguished from google-visualization-barchart and google-charts-barchart due to the current charts/visualization split).
So to make things simple, due to the lack of traffic, just make them all synonyms of each other with google-visualization as the main tag, so that the already low traffic at least gets aggregated and seen by more people.
Note: below numbers are from May, not much has changed since then
google-charts has 1,168 questions
google-visualization has 579 questions
- 107 (18%) are tagged google-charts
- 27 (5%) are tagged google-charts-api
- 10 (2%) are tagged google-geocharts
google-charts-api has 371 questions
google-geochart has 43 questions
- 10 (23%) are tagged google-charts
- 10 (23%) are tagged google-visualization
- 3 (7%) are tagged google-charts-api