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Here's my problem. I have potentially large numbers, anywhere from one hundred to one hundred million or even more. I want to feed these numbers to a chart plotter (http://benpickles.github.com/peity/ if you're curious). the problem is, when you have very large numbers the line chart ends up looking like a flat line since the numbers are so large the differences do not show up on such a small chart. But if you chart numbers like 2,5,8,10,15. you can easily see the chart line going up and to the right steeply.

So in order to plot my mini charts in a way that actually is meaningful, I need to reduce these large numbers into as small numbers as possible BUT maintain a relative difference between the numbers large enough that they plot on a chart well, like with the peity charts. I don't really need a perfect formula (not sure if one exists). If my "large" numbers are growing, I simply want single digit numbers that grow at the same relative pace as the large numbers. If they are some what flat, I want the chart to look flat, etc.

I don't have a strong math background so I don't know if there's an actual math term for this??

Notlikethat
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ijjo
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4 Answers4

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From what I understand in your question, your issue is that the numbers you are plotting have very different scale. If that's the case, you may consider plotting them on a log/log scale: http://en.m.wikipedia.org/wiki/Logarithmic_scale

Mathias
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Look at the function Math.Log(...). I suppose you know it but in case http://en.wikipedia.org/wiki/Logarithm. Logarithm will basically flatter your graph.

Do it for every point before plotting. You can choose by experiments (if its visible enough) the logarithm base. From your description I suppose you should use some big number like 10.

newPlotValue = Math.Log(plotValue, 10); // or Math.Log10(plotValue);
Krzysztof Morcinek
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  • hmm this could be what i need. like u said, i may need to fine tune the value a bit to see which one creates the best values to plot. let me give this one a shot. – ijjo Jul 08 '12 at 05:59
  • Quoting other answer: _like looking at a quick stock chart and knowing that the stock is going up, down, or flat (and with different levels of noticeable steepness_. You can put a NumericUpDown(for log base) next to the plot and redraw the plot after changes of log base. So your users can choose the scale on them own. – Krzysztof Morcinek Jul 08 '12 at 06:05
  • question about choosing the log base. how would i go about dynamically choosing the correct one based on the plotValue? right now if i use 10 for values like 100200, 100300, 100350 you get numbers that are only different in small fractions of a number (5.00xxx for all of them). i need a bigger difference for the chart to show an increase. – ijjo Jul 08 '12 at 06:13
  • If all numbers are like 100200, 100300, 100350 and near then you will dynamically choose base like 2 or even 1. If you have numbers like 2,5,8,10,15 among 100200 (this is actually what you asked about, so I not sure if I am helping), maybe zooming (I mean narrowing x axis range) would do the job. – Krzysztof Morcinek Jul 08 '12 at 06:32
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You only need to decide where is your plot scale.

You could take the smallest as the start of your scale.

The next is to decide a common factor, and show that in your units.

Like if numbers are greater than 1000, divide them all by 1000 and in your label show the scale as (in 1000s). Same can be used for in millions or in billions.

nunespascal
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  • not sure if this is what i need. the charting i'm using doesn't have a scale label. all it shows is a line. the chart's goal is to simply eyeball a direction of the numbers very quickly. like looking at a quick stock chart and knowing that the stock is going up, down, or flat (and with different levels of noticeable steepness). – ijjo Jul 08 '12 at 05:53
  • I am sure you could sure your unit seperately. Stocks are usually plotted best, when you use a multiplication factor. Note that your factor will be different for each scrip. Users are not interested in the factor, that they can see in the price. Deciding the factor depends on the value of the stock. – nunespascal Jul 08 '12 at 06:02
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Suppose you have the following barometric pressures:

double[] p = { 101325.0, 101380.0, 101510.0, 101580.0, 101470.0, 101295.0, 100985.0, };

(numbers in pascals). If you plot them, the graph may look very straight and horizontal because the numbers have all very much the same magnitude.

To better see the changes, simply subtract some fixed number from every term. E.g.

double offset = 100000.0;
var pOffset = p.Select(x => x - offset).ToArray();

Then plot pOffset instead.

In the above example, I just picked the offset 100000.0 "magically". But you could use the minimum of all the values as an offset, that is

double offset = p.Min();

My code examples require using System.Linq;.

Jeppe Stig Nielsen
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