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Comments

Adam

I don't follow the point about the discontinuity at the vertical ray. 24 hours should not necessarily form a continuous line; after all, the two end points are 24 hours apart.

I agree that the circular chart is problematic. Notice the dynamic x-axis labels are physically separate from the chart itself, so it's hard to keep your eyes on both at once. Beyond that, it's simply awkward to mouse over in a circular motion.

It looks like the display is actually modeled on those old style rotary chart loggers (http://za.rs-online.com/web/c/test-measurement/data-acquisition-logging/chart-recorder-accessories/). It is eyecatching. Too bad it doesn't seem to work.

emre

Rotating circles make much sense in a dynamic visualization, which is updated in a sweep, and for periodic phenomena where the ends necessarily meet. For static visualizations of aperiodic phenomena, as considered here, it might be distracting.

Jeff

A line graph would certainly make for easier trend-spotting, but having the circle rotate just means the graph always shows the last 24 hours, with the recent data at the top. If this were a dashboard that someone used to monitor these metrics throughout the day, this would make sense because it provides a consistent place to look for the most recent data.

The discontinuity at the top is just what the previous commenters have said: today's 6:00 data meeting up against yesterday's 6:01 data. However, the fact that the older data actually changes on refresh, rather than simply rotating out of the way, makes no sense at all. I'm betting that's a bug.

jlbriggs

@jeff - I couldn't disagree more about the usefulness of the chart rotating.

The reasons for using a straight line chart over a circular chart are vast and varied (and, hopefully, obvious).

But assuming we are going to use a circle, it should never rotate.

The entire benefit to the circle for a 24 hour data display is that it mimics a clock. The data for 4:00 should always be at 4:00...no other alignment makes sense.

If the latest data points are not immediately obvious no matter where on the circle they are, then we have bigger problems with our chart than even the circle itself.

Jeff

Hi, jlbriggs. I think we're more or less on the same page. Yes, I can't imagine a situation where I would choose a circular chart for something like this to begin with.

My point was just this: if you were creating a display (of whatever shape) that was going to be on someone's desk all the time, and part of their job was to make sure that the things being measured never strayed beyond some defined tolerance, there would be an argument for using the current moment as the fixed point of reference, the better to enable reading at a glance. In that case knowing what was going on at 4:00 is maybe less important than knowing what has happened in the few minutes since you last glanced in its direction. I realize that Facebook's thing is not such a display, but I could see a use for it.

Once of you step out of my hypothetical real-time context, data plotted on a timeline with start and end points that you don't have to think about will likely be much easier to interpret if you're looking at general trends.

derek

Adam, circular charts can be defended where the data are truly circular (for instance, the daily average, over a period of time much larger than a day). What the discontinuity shows us is that, as you say, the last data point twenty four hours ago need not be related to the first data point now.

But that means the data are not periodic! It's a linear time presentation, not a circular one. So there's no reason to curl the graph up in a circle, it should have been a rectangle.

This was Florence Nightingale's mistake too; she represented as a circle that which was a progression, and when she got to the end of a year, she made a second circle :-(

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