Move overlapping pie charts in QGIS

Taming Overlapping Pie Charts in QGIS: A Human’s Guide to Making Maps Make Sense

Pie charts in QGIS? Love ’em. They’re fantastic for showing how different pieces make up a whole when you’re dealing with geographic data. But let’s be honest, things can quickly turn into a visual dog’s breakfast when those pies start piling on top of each other. Overlapping charts become a real headache, hiding the very info you’re trying to showcase and turning your map into an unreadable mess. So, how do we wrangle these unruly charts and create maps that are both informative and easy on the eyes? That’s what this guide is all about.

The Overlap Conundrum: Why It Happens

Why do pie charts overlap in the first place? Well, a few common culprits are usually to blame:

• Too many features crammed together: Think of a densely populated city. If each building gets a pie chart, you’re gonna have a bad time.
• Huge data values, even huger pies: Sometimes, the numbers you’re visualizing just result in massive pie charts that hog all the space.
• One-size-fits-none: Using the same size pie chart for every feature, regardless of its data, is a recipe for overlap disaster. It’s like trying to fit a square peg in a round hole, repeatedly.

Our Arsenal: Strategies for Chart-Taming

Alright, let’s get down to brass tacks. We’ve got several tricks up our sleeves to deal with overlapping pie charts. From simple tweaks to more advanced maneuvers, there’s a solution for almost every situation.

1. Size Matters: Getting Pie Charts Down to Size

• Attribute-Driven Scaling: This is your bread and butter. Instead of a fixed size, let an attribute value dictate the chart’s size. Smaller values, smaller pies – makes sense, right? Head to the “Diagrams” tab in your layer properties, pick “Pie Chart,” and play with the size options. Link it to an attribute, and watch the magic happen.
• Expressions: The Power User’s Secret: Want even finer control? QGIS expressions are your friend. Tweak those attribute values before they determine the size. For instance, the sqrt() function can mellow out the size differences, preventing those gigantic pies from overshadowing everything. It’s like putting the brakes on runaway growth. Try this:
  
  
  sqrt(value)
  
  
  Also, scale_exp() is really useful for transforming an input range exponentially to an output range.
  
  
  I remember one time, I had a map with population data, and the biggest cities had pie charts that were just overwhelming. Using sqrt() tamed those behemoths and made the whole map much more readable.

2. Placement is Key: Finding the Sweet Spot

• Data-Defined Positioning: The Manual Approach: Sometimes, you just need to move things by hand. This lets you drag each pie chart to its perfect spot and save those coordinates in your attribute table. Create “chart_x” and “chart_y” fields to store those positions.
• Then, in the layer properties, under “Diagrams” -> “Placement,” turn on “Data defined position” and point it to your new fields.
• Now, with editing enabled, use the “Move Label and Diagram” tool to nudge those pies around. The “chart_x” and “chart_y” fields will update automatically. It’s a bit tedious, but sometimes, that personal touch makes all the difference.
• Automated Placement Settings: QGIS offers automated placement settings to control how diagrams are placed. In “Position” -> “Automated placement settings” you can enable “Show all labels and features for all layers”, this will draw all charts, even when colliding.
• Centroid vs. Perimeter: Pie charts usually sit smack-dab in the middle of your feature (the centroid). But sometimes, shifting them to the edge (the perimeter) can free up some space and reduce those overlaps. Experiment and see what works best for your data.

3. Leader Lines: Connecting the Dots (or Pies)

• Okay, so you’ve moved your pie charts away from their features. Now what? Leader lines are your visual lifelines, clearly linking each pie to its origin. Without them, your map will look like a bunch of floating pies, lost in space.
• Here’s how to add them: create a new symbol layer in your layer style and choose “Geometry generator.” Set the “Geometry Type” to “LineString / MultiLineString.”
• Then, paste this expression into the generator:
  
  
  make_line( centroid( $geometry), make_point(“chart_x”, “chart_y”))
  
  
  Of course, replace “chart_x” and “chart_y” with your actual field names. Boom! Leader lines.

4. Scale-Dependent Visibility: Zooming In for Clarity

• If your map is digital, take advantage of zoom levels. Show pie charts only when you’re zoomed in enough to see them clearly. This keeps things tidy at smaller scales.
• In the layer properties, under “Diagrams” -> “Appearance,” you’ll find “Scale dependent visibility.” Set the zoom range, and you’re golden.

5. When Pies Fail: Alternative Visualizations

• Sometimes, pie charts just aren’t the right tool for the job. If you’ve tried everything and the overlaps persist, consider these alternatives:
• Stacked Bar Charts: Similar info, different shape. Stacked bars can be less prone to overlap, especially if you orient them vertically.
• Heatmaps: Great for showing density patterns. Heatmaps give you a general overview without individual charts cluttering the map.
• Proportional Symbols: Use circles (or other shapes) to represent the total value, and then use color to show the different categories. It’s a classic cartographic technique for a reason.

6. Solving Overlapping Points

• If you have overlapping points, you might want to display a pie chart with the sum of the values, instead of displaying a pie chart for every point.

Let’s Get Practical: A Step-by-Step Example

Alright, let’s put this all together with a quick example:

The Takeaway: Maps That Communicate

Dealing with overlapping pie charts in QGIS is all about finding the right balance. It’s a mix of technical skills, cartographic principles, and a little bit of artistic flair. By using these techniques, you can create maps that not only look great but also communicate your data clearly and effectively. So go forth, tame those pies, and make some awesome maps!