Fix Accessibility metric diagram

The problem with the diagram was that the data input exceeded the d3.js
configuration. My solution thins out the values. I am not completely
happy with the solution, since it can only be an intermediate solution.
With the current implementation it is still possible that there are 100
values which might be too much.

With all probability I have to think about an alternative way to make
the metric results accessible. In addition this commit also implements
the normalization of the Accessibility results. This commit fixes #1.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Replace field type for fetching metadata

Consequently I have changed the type which defines the field by which
the metric results are grouped to symbol. Before the type was string.
Hence, some type conversions were added.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>