I have a strip chart which is constantly accepting data. I have been displaying it like this:

Data is added to the right

Under this design, data is added to the right edge, and old data "falls off" the left edge, but it can be seen again by making the window larger. This design is inspired by the Windows Task Manager:

Did I just give away my hardware specs? :)

Today, one of my coworkers suggested that I change the design, instead displaying the data like this:

Data is added to the left

In this design, the right edge is static, and data is added to the left side. When the data reaches the left edge, the graph shifts to the right in a paginated fashion.

I cannot rely on the expertise of my users, nor on their experience using computers. They will be interested in seeing the history, for instance, leaving the device on over the weekend to collect data, then coming in on Monday and reviewing it. The data comes in roughly once a second, so long periods of time can lead to lots of information for them to process. How much is contained in a single screenshot depends a few different factors (such as monitor resolution), but can come out to anything between one or two minutes to half an hour.

Which of these two designs would be more useful to them?

  • The big question is: how much data do they need to reach their goals? You say, weekend, okay, but what does it mean screen-wise? How frequent is the sampling, is this screen a day's worth? Is the weekend the only and main use case? Or perhaps it's a day's worth of data, which they look normally daily? Do they need to compare with historical data to be meaningful, eg. data from last day, last year this day, etc? Do they need to check longer trends, eg. a month? What is this data used for? – Aadaam Aug 17 '12 at 16:32
  • @Aadaam The users will definitely want to analyze historical data to identify long-term trends, but also to diagnose short-term anomalies. Really, though, the meaning of the data is very situational. Our product is a measurement tool. – gobernador Aug 17 '12 at 16:42
  • I don't get why data would grow to the left as in the second example. That would mean newer data is to the left, correct? That doesn't make much sense in a Right to Left language context. – Ben Brocka Aug 17 '12 at 16:45
  • @Ben The new data would be on the left. I suppose it could grow in the other direction, we would decide that when (if) we end up with that display scheme. The question is would the "static edge with pagination" or the "push the old data off the cliff" method be better – gobernador Aug 17 '12 at 16:50
  • At that point it depends which is more important; are users wanting to look at current data and maybe look back, or are they wanting to see where the data started/where they left off, then, maybe, look at later data? – Ben Brocka Aug 17 '12 at 16:57

Let's search for an analog metaphor: the seismograph.

Seismographs are those little devices that show when there's an Earthquake coming. Their operation principle is simple: there's a piece of paper which is moving forward, and there's a pen, which is fixed on a spring. When the Earth shakes, the pen moves, causing the pen to move sidewise.

There are two kinds of seismographs available: one is the strip-based seismograph, the other one is the drum-based seismograph.

An analog strip seismograph

Simple analog seismograph, possibly hand-made for demonstration purposes. The paper has to be dragged continously either manually or by some mechanism (Image credit: Fisher Scientific / Hubbard)

An analog drum seismograph

Analog drum seismograph (image credit: Earthquake, Foundation and Concrete Engineering Blog)

In practice, the drum seismographs are used in field today, as they're reusable. Their drum is essentially a whiteboard bended, and there's a mechanism which moves the writing head to the next column after each round.

Now with these metaphors in mind, let's see what can we do.

Metaphors for a strip seismograph:

In a strip seismograph data is recorded on long strips. You have the following options to look up historical data

  • you either simply look back on the long strip (using a scrollbar), especially if the scroll is theoretically infinite
  • you take a roll of strip out of the storage and read it (if it's not infinite)
  • or you grab a part of the strip in your hand while it's still writing.

strip seismo - scroll

strip seismo - hold

Metaphors for a drum seismograph:

With a drum seismograph, you have two options:

  • show the data from the previous day(s) in columns below (or above). Make sure that the right hand of the screen is always the current moment (so, you can compare data from half an hour ago to data from 24.5 hours ago simply by looking at the vertical line)
  • show previous day's data with a different, less prominient color (like, gray), while today's data is overwriting it.

The benefit for the latter that it doesn't need that much space, the problem is that it could be only used to show 2 cycles (let's say, 48 hours) of data.

drum seismo - columns

drum seismo - overwrite

Hope this helps.

(Hand credit: Luzbonita)

  • +1 for the analogies. Another one that comes to mind, and is electronic at that, are the monitors used in Intensive Care Units. IIRC (I wasn't there because I was healthy) new data came in on the right and the trace fell off the left edge. And these are of course inspired by the paper trace ECG's which also have the stylus on the right adding new data while the paper with the trace rolls left. Don't think I have ever seen it the other way around... – Marjan Venema Aug 17 '12 at 20:05

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