How can I move diagrams?

Question

Is there a way to locate diagrams and move them wherever you want in QGIS 1.7.4/ 1.8.0? I created a pie chart under the tab "diagram". I can mark those diagrams with a green box and move them via the label-tool, but they won't change their position in the end. When I create diagrams with the "diagram overlay" tab they cannot even be marked.

Also, what is the difference between "diagram" and "diagram overlay"?

Answer 1

If your diagrams represent values from polygons, here's what I do: Create a new shapefile of centroids ("Vector->Geoprocessing Tools->Polygon Centroids"). This is a point layer with all the data columns from the original polygon. Now edit this point layer, and move the points to wherever you want each chart. And finally recreate the charts from this point layer. THey will appear at each point's location, not necessarily at the actual polygon centroid.

If the charts are from points to begin with, and you still need to move their location, I'd suggest to make a duplicate point layer just for the charts. Edit this duplicate and move the points as necessary. Not very elegant, but it'll work...

Answer 2

I'll try to make the mentioned approach more clear. The approach works with the polygon geometry features.

Diagrams is just the graphic overlay to a vector layer. It does not have any geometry or feature properties. After creating diagrams, you still work with the same layer, but with the data styling.

While using diagrams you often meet the overlapping problem. You may try to use different options in 'Placement' menu in Diagrams Toolbar, however it not always help.

The most common way to place diagrams would be 'Over Centroid', so it saves the spatial reference to every feature. However, it in 90% of cases you would meet the overlapping problem. @Micha proposes to move just some diagrams instead of trying to use other 'Placement' options which usually don't save the direct spatial reference: "let's build diagrams not for polygons, but for points which have the same position and attributes, and if some diagram overlaps, we would move it a bit within its polygon, so it would save the spatial reference".

There are following steps:

1. Create polygon centroids (Processing Toolbox -> Polygon Centroids). We are getting new shapefile with points. Ok, we have got points with the same data as polygons have and they are in the position of the geometry centre of each feature (exactly where diagrams with 'Over Centroid' placement would be rendered)
2. Create diagrams for new (points) shapefile with 'Over Centroid' Placement. Choose the 'No Symbol' option in the Style toolbar, so nobody understands our "trick", in case you're using some transparency for the diagrams (you still may need it for moving your features).
3. Move your points features for creating your map with no overlaps just by editing your points layer. Not the most representative screenshot, sorry.

---

Be carefull while working with polygons with curve geometry or ones with the 'ring' geometry for not confusing the appropriate point. You may use labels for be sure you're using the correct point to move.