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Learn how to convert pixel/scene coordinates from mouse clicks to graph coordinates in PyQtGraph. This guide will help you implement an effective solution in your PyQt5 applications.
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This video is based on the question https://stackoverflow.com/q/67844662/ asked by the user 'yurumi' ( https://stackoverflow.com/u/16132256/ ) and on the answer https://stackoverflow.com/a/67844894/ provided by the user 'eyllanesc' ( https://stackoverflow.com/u/6622587/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Converting Pixel Coordinates to Plot Coordinates in PyQtGraph

When working with graphical applications, you may find yourself needing to translate the pixel coordinates obtained from user interactions into meaningful plot coordinates. This is particularly relevant when utilizing libraries like PyQtGraph in PyQt5, which allows for rich visualizations. In this guide, we'll tackle a common problem: converting coordinates from mouse click events to graph coordinates, ensuring you can accurately determine where a user has clicked on a graph.

The Problem

Imagine you have a GraphicsLayoutWidget containing a PlotItem, and you want to register a click event anywhere on the graph — not just on curves or graphical items. With the mouse click, you can obtain position coordinates that look something like this:

[[See Video to Reveal this Text or Code Snippet]]

However, these pixel coordinates are not very helpful if you want to translate them into normalized graph coordinates (like (0.3, 0.5)). The challenge is finding a suitable way to map screen coordinates to plot coordinates effectively.

The Solution

Step 1: Setting Up the Environment

To start off, ensure you have pyqtgraph installed in your Python environment. You can install it using pip:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Creating the Main Window

We will create a PyQt application with a GraphicsLayoutWidget containing a PlotItem. Here's a simplified structure of our main application:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Capturing Mouse Click Events

Next, we will implement the _on_mouse_clicked method that will handle mouse click events. From the mouse event, we need the scene position, which we can then convert to view coordinates.

[[See Video to Reveal this Text or Code Snippet]]

How It Works

sigMouseClicked Signal: This signal is connected to our custom method _on_mouse_clicked, allowing us to respond whenever a mouse click event occurs within our GraphicsLayoutWidget.

Coordinate Conversion: The key function here is mapSceneToView, which converts the scene position of the mouse event to the view (plot) coordinates based on the axes of your graph.

When you click anywhere on the plot, the coordinates will now be printed in terms of the graph itself.

Conclusion

By following these steps, you now have a fully functional method for converting pixel or scene coordinates to plot coordinates within a PyQtGraph project. Such functionality enhances the interactivity of your visualizations and allows for more precise user inputs.

This guide showcases how you can streamline the process of obtaining useful graphical data, ensuring that you are well-equipped to develop interactive applications using PyQt5 and PyQtGraph. Happy coding!