Plotter: a tool to create bitmap charts for the web

21 December, 2009 (19:29) | data publishing, data visualization, tips, web sites | By: jerome

In the past couple of months, I have been busy maintaining a blog for OECD: Factblog.

The idea is to illustrate topics on which we work by a chart which we’ll change regularly. So in order to do that, I’d have to be able to create charts of publishable quality.

Excel screenshots: not a good option

There are quite a few tools to create charts on the net. Despite this, the de facto standard is still a screenshot of Excel, a solution which is even used by the most reputable blogs.


This is taken from

But alas, Excel is not fit for web publishing. First, you have to rely on Excel’s choice of colours and fonts, which won’t necessarily agree to those of your website. Second, you can’t control key characteristics of your output, such as its dimensions. And if your chart has to be resized, it will get pixelated. Clearly, there is a better way to do this.

That's a detail of the chart on the link I showed above. The letters and the data bars are not as crisp as they could have been.

That's a detail of the chart on the link I showed above. The letters and the data bars are not as crisp as they could have been.

How about interactive charts?

Then again, the most sensible way to present a chart on the web is by making it interactive. And there is no shortage of tools for that. But there are just as many issues.
Some come from the content management system or blogging environment. Many CMS don’t allow you to use javascript and/or java and/or flash. So you’ll have to use a technology which is tolerated by your system.

Most javascript charting solutions rely on the <CANVAS> element.  Canvas is supported by most major browsers, with the exception of the Internet Explorer family. IE users still represent roughly 40% of the internet, but much more in the case of my OECD blog, so I can’t afford to use a non-IE friendly solution. There is at least one library which works well with IE, RaphaelJS.
Using java cause two problems. First, the hiccup caused by the plug-in loading is enough to discourage some users. Second, it may not be understood well by readers:

This is how one of my post reads in google reader.

This is how one of my posts reads in google reader.

And it’s futile to believe that readers will read blogs from their home pages. So if all readers can’t show it well it’s a show-stopper.

A tool to create good bitmap charts

So, in a variety of situations the good old bitmap image is still the most appropriate thing to post. That’s why I created my own tools with Processing.

plotter windows

plotter mac OS X

plotter linux

Here’s how it works.

when you unzip the files, you have a file called “mychart.txt” which is a set of parameters. Edit the file according to the instructions in “instructions.txt” to your liking, then launch the tool (plotter application). It will generate an image, called “mychart.png”.

The zip files contain the source code, which is also found here on my openprocessing account.

With my tools, I wanted to address two things. First, I wanted to be able to create a chart and to have a precise control of all of its components, especially the size. In Excel, by contrast, it’s difficult to control the size of the plotting area, or the placement of the title – all of this things are done automatically and are difficult to correct (when it’s possible). Second, I wanted to be able to create functional thumbnails.

If you have to create smaller versions of a chart from a bigger image, the easiest solution is to resize the chart using an image editing software. But that’s what you’d get:

That's the original chart.

That's the original chart.

And that's the resized version. Legible? nah.

And that's the resized version. Legible? nah.

But what if it were just as easy to re-render the chart in a smaller size, than to resize it with an external program? My tool can do that, too.

Left: resized, right: re-rendered.

Left: resized, right: re-rendered.

Here’s a gallery of various charts done with the tool. The tool supports: line charts, bar charts (both stacked and clustered), dots charts and area charts. No pie charts included. It’s best suited for simple charts with few series and relatively few data points.

Impact of energy subsidies on CO2 emissions

Impact of energy subsidies on CO2 emissions

Temperature and emission forecasts

Temperature and emission forecasts

Greenhouse gas emission projections

Greenhouse gas emission projections

I hope you find it useful, tell me if you do and let me know if you find bugs.


Comment from Jon Peltier
Time December 22, 2009 at 5:37 am

1. Part of the reason the images from the appleblog site are of such poor quality is that they used the JPG format, which was designed for photographic images with continuously varying tones and shades. A chart has areas of constant color, which change abruptly to areas of different colors. Formats like GIF and PNG render sharp color changes, such as text, clearly, while JPG smears out sharp edges and text.

2. Regardless of which tool you use to generate web graphics, the original source of the graphics should remain in vector format until it is finally sized to the exact size and shape that is required for its use on the web. It should not be rasterized until it is iin its final configuration.

Comment from jerome
Time December 22, 2009 at 12:48 pm

It’s true that saving a chart in jpg format, especially with less than perfect quality, will smear it terribly.
On a couple of occasions I’ve seen my charts reposted on other blogs in jpg format and the result is less than optimal.
For the vector vs raster problem, I think in all fairness it’s possible to create a crisp chart out of Excel if all operations are performed in an order that make sense, for instance, by only redimensioning a vector object rather than a bitmap. But that’s seldom the case, partly because some CMS resize and compress images automatically when they are posted.

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