[Qgis-developer] Improving the legend for map-units, data-defined symbology, diagrams and labels

[Andreas Neumann]

Hi,

We need to solve a number of open issues with the legend - preferably in 
the next QGIS release. This concerns mainly the print composer legend, 
but perhaps the legend in the legend tree could be solved at the same 
time. At the same time we should also think how these improved legends 
could be exposed in QGIS server.

I am trying to list the things I would like to see improved:

1. Support for legends for graphical properties defined with map units. 
Currently these legends display in the print composer at very strange, 
unpredictable and unusable sizes. E.g. a stroke-width or dash defined in 
map units is either way too big or too small. Same for symbol sizes 
defined in map units. We need a way to translate these map units in 
usable paper units suitable for display in the legend. Let the map 
author define a corresponding mm size (along with the map units) - 
solely for the display of the legend.

2. Display of legends for graphical properties defined with data-defined 
symbology. We need a way to specify common graphical properties in the 
legend that are of interest in the legend. F.e. - if I define the colors 
data-defined I would like to be able to specify the expected colors that 
may appear in my data/project. This could be either done dynamically (by 
scanning the data) or statically (decided by the map author). Perhaps 
both methods. To me, the static method may be more predictable and more 
performant.

3. Legends for diagrams (pie charts, bar charts) - something similar to 
http://esri.force.com/servlet/rtaImage?eid=087300000008Nki&feoid=Body&refid=0EM30000000Co0O 
- showing both sizes and colors - nicely stacked into each other.

4. Legends for labels - here we also have the same issue: how can we 
display a label legend where graphical properties are set based on 
data-defined values?

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I know this is not yet well specified - but I would like to open the 
discussion how we can solve the above issues. And I am looking for a 
QGIS developer who is interested in solving the above issues. If you are 
interested in developing above listed improvements, please join the 
discussion or contact me offlist.

Do you have ideas how to solve the issues with map units and 
data-defined symbology?

There is at least one other german organization (QGIS user) who is 
interested in this and would help finance this effort. Perhaps we can 
find more organizations who would help finance.

Thank you for joining the discussion,

Andreas