There are a lot of cool visuals but many of them are almost unusable because it's incredibly difficult to guess what they are expecting as input. To take one example Microsoft's own Histogram visual has a 'Values' and 'Frequency' input. There's no guidance at all to suggest how these should be used. Many other visual suffer from this, including the standard ones (I _still_ can't sense of the Scatter chart naming).
I'd like to humbly suggest that just to the right of the Fields/Formats/Analytics tabs, there is a small help icon which can be used to open a link/embedded resource that gives some documentation for the selected visual.