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Tick marks in graphs

Although this is probably a trivial concern, it nevertheless engendered many long arguments amongst my colleagues and myself in graduate school*.

When rendering a graph of one's results, where ought the tick marks be placed: inside, outside, or both? I have always felt their proper place is inside the graph's axes, purely upon aesthetic grounds. Perhaps someone here would have insight on the matter.

*This is a well known phenomenon. Often the most trivial of issues prompt the most passionate discussions amongst students.

-- Tom Udo (email)

If you get rid of the axes then the question will go away. Chapter 6 of VDQI has many examples of alternate uses for tic marks and axes.

-- Dave Nash (email)

Graphs are best at showing overall trends and the big picture. That is when they are dramatic and memorable. They are less successful at showing detail. If your readers need to see the detail — if they need the numbers, or indeed tick marks or gridlines, so they can work out the numbers - then give them a good table, not a bad graph.

My evidence for this is, of course ET but also Ehrenberg and British Standard 7581:1992 The Presentation of Tables and Graphs. (There is no ASA equivalent, as far as I know. Does anyone else know?)

-- Sally Bigwood (email)

Arguments for inside: When resizing graphs, it helps to scale axes to the same size (particularly useful when constructing small multiples).

Arguments for outside: 1) Depending on the data, tick marks inside the axes could conceal or be mistaken for the data. 2) It places the tick closer to the label, number, or other indicator.

-- Zen Faulkes (email)

There is not a generally correct answer.

If tick marks and axes are worthwhile (and often, they are not), I prefer short, thin ticks placed outside of the data area: I like the ticks to be closely assoicated with the numerical labels. One nice solution is to have major ticks cross the axis, and minor ticks only outside. This works especially well with logarithmic scales.

-- Alex Merz (email)

If the axis are being used a visual barrier then the outside should be chosen. A case for that would be a bar graph where the ticks may overlap more important ink inside the axis area. In such an example the ticks may just be pointers to names for each bar in the graph. Think of your axis as a bucket. For example, it is more helpful to have the name of the contents of a bucket labeled on the outside of the bucket than the inside of it.

If the axis are themselves data rich, the inside seems appropriate. An example of a data rich axis may be a timeline, or distance scale. For example it is more helpful to have a ruler on the inside of a bucket full of water to know how deep the water is than to have the ruler on the outside.

If you can't decide where to place the ticks on your bucket based on what is inside your bucket already, you probably don't need the bucket at all and you could ugly it up further by making it more redundant by placing the ticks inside and outside! Please dump out your bucket and start over if you do this.

-- Jeffrey Berg (email)

William S. Cleveland, in his excellent book The Elements of Graphing Data, recommends that tick marks should point outwards ‘because ticks that point inward can obscure data’. See the discussion on pages 31–35, and especially figure 2.12 and 2.13.

See also Cleveland’s discussion on the number of tick marks, on pages 39–41.

-- Karl Ove Hufthammer (email)

The short answer is to gray down the axes, their tic marks, and the grid so that they don't clutter up anything.

The full analysis, including a long discussion of Bill Cleveland's work, is found in Beautiful Evidence, pp. 118-121. See also Envisioning Information, chapter 3 on layering and separation.

-- Edward Tufte

I see there will be a new way of charting on the Apple in the near future.

-- Tchad (email)

Box off, Tick out

-- TheTick (email)

Threads relevant to statistical graphics:

Sports data (along with financial and medical data) are an obvious and natural application of sparklines.