Google Charts API use

Hello, I'm testing Google Charts API plugin for rails and have a few
questions about it's use (I searched for a developper documentation
for about an hour without founding anything really interesting):

Here is my test code:
# Testing google charts api
chart =
chart.type = :bar_vertical_stacked
chart.height = 400
chart.width = 600 = [@totalAmount, @totalPercent]
chart.colors = '346090'
chart.labels = ["total", "percent"]
@small_sales_chart_url = chart.to_url

And in the view:
<%= image_tag @small_sales_chart_url %>

The generated graph url is:|percent&amp;chd=t:300.0,100.0

This is great, but I would like to know how to:
  - Set the space between each bar
  - Define the y axis labels
  - Create a bar with values that can be negative
  - Write the value of each bar directly on the graph
....and so many more.

If anyone knows where I could find a complete developper guide that
would be great, if not maybe you can help me with those few questions
in the meantime.