A blog about exoplanets, scientific computing, and life as I know it.

Data visualization in Cocoa

Probably one of the best advantages of an interactive GUI for scientific programs is the ability to visualize data in real time. While you could as well output a full dump of your data and plot with an external program, there is great pleasure in exploring how your model reacts to mucking and futzing around with the parameter space.

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In the Systemic Console program (above), I wrote a flexible Swing component for plotting. As you can see, it takes care of plotting scatter (with optional error bars) and line plots, histograms and 3D line plots. Plot styles are easy to customize and are close to most journal styles by default. The component takes care of printing, zooming, tracking the mouse pointer and has a customization panel. Writing this component myself and optimizing it for quick redrawing took quite a bit of time. The graphical performance of Swing under OS X was often under par, especially when trying to get antialiasing right. There is no built-in way to generate PDFs (I ended up using the excellent iText for that). And finally, as usual a lot of the tedium derived from Java’s verbosity. What’s available for Cocoa?

Plotting with Cocoa

I’m still quite a way to be worrying about the UI of my new application; currently, I am mostly thinking about the overall framework, how much of it to write in C and how much to wrap in shiny Objective-C classes. However, I’ve spent a bit of time researching the current landscape of data visualization. I will survey some of the options below, along with some IMO pros/cons.

1. SM2DGraphView

Website, Documentation, Open source
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An open-source framework used by a few Mac applications. It can show line and scatter plots, and pie charts. It is very well documented and easy to use (see e.g. this tutorial at MacResearch).

  • PROS: Easy to use, well documented, open source
  • CONS: Plots look outdated, difficult to get output in line with journal standards, low quality output compared to other options

2. CorePlot

Website, BSD license
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I have only had time to give a cursory look to this framework, but from what I have seen from glancing at the examples and the documentation, it seems like a very complete and decently documented framework. The project is frequently updated, and there seems to be a good amount of tutorial and forum posts on Stack Overflow about it. This is mainly due to its compatibility with iOS.

  • PROS: Complete documentation (including a .docset for Xcode), plots look good on screen
  • CONS: None, but it seems more complicated than other options.

3. DataGraph framework

Website, Free for open source projects, $400 otherwise (including in-house projects)
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This is basically the full plotting component powering the excellent DataGraph. The framework is available for free for open source projects.

The philosophy is very different from the other components listed above. A plot is defined beforehand by creating a template in DataGraph, rather than programmatically in the code; this is similar to how Aqua UIs are usually serialized in nib files designed with Interface Builder rather than created programmatically. This makes setting up a static plot almost trivial. You can also connect sliders, color wells and other interface elements with plot and formula parameters extremely easily. Zooming, panning and exporting are taken care by the framework. Finally, the output quality is the same as DataGraph, and therefore excellent.

  • PROS: Quality is awesome, free for open source projects, trivial to get up and running with a pre-made DataGraph template
  • CONS: As far as I can tell, you can’t add new plots programmatically, so you might have to allow for several plots in the template and dynamically show/hide them. Documentation is very scarce.

4. A custom NSView

This might be easier than it looks, and is basically the path I took for the Systemic Console to get exactly the look and functionality I needed. It has the advantage of implementing exactly what you need without unwanted baggage.

An object inheriting from NSView will do its drawing in the drawRect: method. The actual drawing can be performed using NSBezierPaths to draw lines, ovals, rects and rounded rects with quality antialiasing. Finally, PDF creation is handled by Cocoa automatically (see, e.g., this).

  • PROS: Completely customizable presentation, PDF export for free, probably the best solution if only handling a few uncomplicated plot types.
  • CONS: Writing everything yourself, meaning time-consuming to get right. NSBezierPath not available on iOS.

Conclusions

This is just a quick and undoubtedly incomplete survey of options for plotting with a Cocoa view. For my project, I will probably go with #3 or #4, with Core Plot a close third and the best option for iOS development. The DataGraph framework is truly impressive for output quality and breadth of options, and trivially easy to set up for static plots.

Please let me know if there’s any library I missed in this post. You can leave a comment below or send me a message with the “Ask anything” link at the top of the blog.