Ok, so about ten days ago I challenged my readers to solve the following charting problems with the Google API - Bullet Chart, Sparkline and Square Pie. I provided a sample bullet chart but asked if anyone could improve upon it. Unfortunately, only one brave soul, Jeff Donnici, took up the challenge and provided two solutions for Sparklines.

I am inviting Jeff to write his own discourse on how to create sparklines with the Google API, so Jeff, please email me (my email is at the bottom of the right-hand nav) with your description of how to create the sparklines.

First, let’s cover some basics of the API.

The API is pretty simple, it’s just a URL with a bunch of query parameters attached to define the chart and its elements. It can also be a bit annoying when you’re encoding the data sets and their associated information. My guess would be that to really get the power out of the API you would want some sort of programmatic interface to it as building the chart request ‘by hand’ can be laborious.

So, components that are used in these examples:

• Base API URL: http://chart.apis.google.com/chart?
• Required Elements
  • Chart Type Parameter: cht - Allowable Chart Types
  • Chart Size: chs - Chart Size in pixels [w]x[h] (chs=250×250)
  • Chart Data: chd - Chart data encoding methods (simple, text, extended)
• Optional Elements Used
• Chart Title: chtt - Text Title for the Chart
  • Chart Title Text Size: chts - Title font size and color
  • Bar Chart Height: chbh - Bar width
  • Chart Colors: chco - data series colors
  • Chart Background Fill: chf - Chart fill options

For me, let’s start with the Bullet Chart. The stumbling blocks here are that

1. The Google Chart API does not support combination charts, or at least not a bar chart / scatter plot combo, which is how I create Bullet Charts in Excel.
2. The API does not seem to support single data point properties. In other words, I can set the color and width properties for the stack (e.g. series) but not for data points in the stack.

If you remember the Excel example of the Bullet chart, you will recall that we have 3 data series (top, middle, bottom) each with 4 data points to create the basic visualization of the Bullet chart. We then combine into that chart a scatter plot to create the target line. Well this API doesn’t support coloring for a single data point (which is required in the Excel solution) so what does Google offer as a replacement? Well, with the background fill, we can go a linear stripe that will take up some percentage of the chart space - ok, I think I can work with that:

But first things first. Let’s define the chart type:

http://chart.apis.google.com/chart?cht=bhs

• cht=bhs tells Google that we are working with a horizontal bar chart

Next, let’s set the chart size

http://chart.apis.google.com/chart?cht=bhs&chs=250×40

• Remember, chs wants [width]x[height] in pixels and it’s required

While we’re at it, lets go ahead and set the bar width (or height if you prefer)

http://chart.apis.google.com/chart?cht=bhs&chs=250×40&chbh=10,25

• chbh lets us define that bar width and optionally the spacing between bars. In this example the gap is set to 25 to make sure that the chart fill area shows up around the bar itself.

Ok, for the last of the easy bits, let’s go ahead and set the Chart title

http://chart.apis.google.com/chart?cht=bhs&chs=250×40&chbh=10,25&chtt=Homepage+Retention+Rate:+90%

• Note the plus (+) symbols in the Chart Title text, this tells the API to use a blank space when creating the title

On to the data. As noted above, there are 3 options for encoding data - simple, text, and extended. I haven’t really figured out why you would use simple because it’s basically a substitution scheme that feels more annoying than anything. However, in this example we are using text encoding because it allows us to use regular numbers with up to 1/10th precision.

http://chart.apis.google.com/chart?cht=bhs&chs=250×40&chbh=10,25&chtt=Homepage+Retention+Rate:+90%&chd=t:90.0

• when adding the chart data, you must first tell chd how you are encoding the data (chd=t) and then supply the data itself (:90.0)

The above gives us a basic bar chart:

Now we need to add the elements that will make this look like a Bullet chart and we’re going to get it all done with chart fill!

http://chart.apis.google.com/chart?cht=bhs&chs=250×40&chbh=10,25&chtt=Homepage+Retention+Rate:+90%&chd=t:90.0&chf=c,ls,0,000000,0.50,cccccc,0.25,000000,0.005,efefef,0.245

chf (chart fill) takes the following arguments:

• fill area: background (bg) or chart area (c), in the Bullet chart, we use the chart fill method
• fill type: solid (s), linear gradient (lg), and linear stripes (ls)
• angle: angle of the stripe between 0 (horizontal) and 90 (vertical)
• stripe color and width pairs: color hex value and length expressed as a percentage

If you look at the URL snippet above in bold, you will see that we have defined

• the background element as "chart fill" (c)
• the fill type as "linear stripe" (ls)
• the "angle" as zero
• The first section is black and extends to 25% of the length
• The second fill section is the same as the first
• the third section is also 25% but is a dark gray
• the fourth section, only 1/2 of 1 percent in width is black - It’s a trick, this is the target line!
• The last section is 24 & 1/2 percent of the length and is a very light gray

That’s it, now we have a functioning bullet chart

I sure wish that I could remove the axes lines, and add data labels to the bar and target line, but oh well.

Ok, I need to take a break as this is quite a long post. In part 2, we will walk through the pie chart solution (which is not that great),

I’m not quite sure how long I’ve been ignoring the Google Chart API but I saw it in a post the other day which reminded me, or at least gave me the inkling, that I had seen some discussion of it before.

Well, that was several days ago and I finally had some time to play with it and in a couple of hours created the following bullet chart:

So here’s the challange

1. Create a Better Bullet Chart than the Above
2. Create a Sparkline
3. Create a Square Pie Chart

Using the Google Chart API of course.

Once you’ve created your solution, post a link to it here in the comments. For the solutions that we like best, I’ll ask the author to provide a description and instructions to be published here in a follow up post.

Update: 12/15/2007

Jeff Donnici sends us two sparkline examples.

Simple:

With Banded Background:

Update 12/19/2007

Well with only Jeff taking up the challenge after 10 days, here’s the final solution: a so-called square-pie chart. Of the 3 types this is the most poorly supported by the Google API mostly because on bar charts you cannot control the scale of the axes.

Update Feb 29, 2008

Geoff offers us this austere Bullet Chart solution:

And this slightly "dressed up" version:

Update: Google Analytics Team, please don’t do this anymore. I just turned on the ‘compare to past’ feature and noticed that you are doubling up series on the sparklines. This is a big no-no as far as I am concerned because the sparkline with mutliple series on it just ends up looking like a squiggle. The data from the second series actually becomes chart junk!

If your a visual analysis wonk like me you probably couldn’t help but be excited with the new version of Google Analytics and the introduction of Sparklines. To me, it comes across as a major endorsement of sparklines as a business tool.

But, for this implementation to be a useful endorsement of sparklines, they have to show value as a tool and I don’t think the GA implementation is quite there - yet. Remember the new version of GA is in Beta so the team has lots of opportunity to improve.

From the Visitor Summary Report…

In this case, the shaded area is not useful, it’s distracting. The reason for this is relatively simple - it’s shading the area under the curve (plotted line) in the case of a sparkline, the shaded area should code more or different information. I’d really like so see this shaded area represent some UCL and LCL data, for instance the standard deviation (plus or minus) from the mean of all values.

The value currently associated with the sparklines are the total or total average for the period represented when they should be the current (or most recent) value.

I’d like to see some data markers on the chart - current value, high water mark and low water mark. Of course, the markers should be color coded to their value on the right. So, for instance the most current data point marker could be red and the current value would also be in red. The high and low water marks would be different colors and their associated text would be those colors to tie them together.

If that’s as clear as mud, here’s a quick example:

Notice that my example is a bit longer, this is because Tufte recommends shooting for a 45 degree angle on the line slopes (it eases understanding)

1. The gray area represents the ‘normal’ distribution of visitors - the "biorhythm" as Avinash would say and it is the mean +/- 1 Standard Deviation
2. Both the oldest point (starting point) and newest point (end point) are marked in red
3. The watermarks (high and low) are marked in green
4. For each of the data points that are marked (except for the starting point) an associated value is presented to the right and is color coded so that associations between the chart and the values are easily made

Finally, I’d like to see the sparklines be more contextual to the report or dashboard that they are in. For instance, I’m not quite sure why total page views is reported in the Visitor report. Also, is that average page views per visit or page views per visitor? I’d rather see average visits per visitor and average usage days per visitor (daily visitors divided by absolute unique visitors) than some of these other metrics.

What do you think of the Google Analytics sparklines?