I really dislike stacked bar charts… Let’s see a new bad example.
Steve Rubel shares with us how he spent his time online over the last two weeks. He uses the Firefox add-on PageAddict to monitor the time. He writes:
As you can see almost all of my time online is work related. Still I can see that I need to shrink my social network usage a little bit and increase my time with documents, web applications while also keeping RSS contained. I also need to go through the undefined section to see if there are big groups of sites that can be tagged.
I would say that it is hard to see what Steve Rubel wants us to see. It is not his fault, of course, he is just pasting a chart from the application. I do see something interesting: since he spends “the vast majority of [his] computing time” using Firefox (let’s say 80%) he’s using his computer around 4.5 hours a day only. That’s nice… 
If I wanted to visually track my time online (I should…) these are some of the options I’d like to have:
- Color-code work/non-work related categories;
- Label the x axis with dates, not “days ago”;
- Remove non-working days;
- Use small-multiples to track each category;
- Use weeks instead of days;
- Annotate outliers;
- Show planned vs. actual time spending;
- Minimize the “undefined” category;
I’m installing the add-on. Hope I can have an interesting dataset to share by the end of October.
Do you use these tools? Do you like their reporting functionalities?
These are 10 basic tips for column and bar chart design:
- A column chart is not a skyline: if you can’t see the individual patterns, consider removing some series or create several smaller charts;
- If you are charting categorical data sort the columns; if there is more than one series, allow the user to sort the data herself;
- If you are displaying time series, column charts are not interchangeable with line charts: column charts allow you to compare individual data points, while a line chart shows the trend; be sure to select what your audience wants to see;
- For target/actual series (like budget/actual) overlap them but make sure they can’t be taken for stacked bars; you can do it by using a different column width for each series or by setting filling to none (usually the target series);
- Use horizontal bar charts when x labels are too large to be correctly displayed;
- The y axis scale should start at zero; this is particularly important if you are using bar charts; make sure you have a (very) good reason to break this rule;
- If you really need to label each column try to minimize its impact; in Excel 2003, select Format Data Labels / Alignment / Label Position: Inside Base;
- Don’t use multiple colors for a single data series;
- Avoid stacked bar charts;
- Use category/subcategory to label the x axis. For example, instead of having Mar-2008, Apr-2008… use Mar, Apr and place 2008 in the second line.
As usual, feel free to add your own tips in the comments.
This chart [via Junk Charts] in the New York Times uses a “tornado” chart (a population pyramid-like chart) to display two series, advertising spending in measured (traditional media) and unmeasured (Internet…) channels.
When discussing how to create population pyramids, I wrote that I don’t really like tornado charts, specially if you only have two series, because it takes up too much space and you can’t compare each entity (age group or, in the example below, corporations).
I would select a bar chart with overlapping series, like the examples below:
Both clearly show the ranking by measured channels and the proportion of advertising spending in the unmeasured channels. Also, it is a safe format for the target audience.
The role of the sorting key
The original chart sorts corporations by advertising spending in the unmeasured channels. The sorting key depends on what you want to say, but you should play with the different options before selecting the final solution. As a rule of thumb, a bar chart should be sorted by the series with larger values and/or the highest variability. Sorting this chart by measured channels one can easily spot the higher proportion of spending in the unmeasured channels at the bottom. If “graphical excellence is that which gives to the viewer the greatest number of ideas” (Tufte), then in this case you should select the “measured” series as the sorting key.
Take away messages:
- Know your audience and select a design with those readers in mind;
- Be sure that the reader can easily compare what you want him to compare;
- If the overall shape of your chart depends on a sorting key, try several options and select the one that shows the most interesting patterns;
- Don’t be afraid of repeating the same chart with different sorting keys (don’t overdo): each one has a unique view on your data;
[Update: you don't need to repeat the same chart with different sorting keys if you can interact with it, like in the basic spreadsheet I used for these charts. I added also a small stacked bar to display proportions.]
