choosing a y-axis scale

[SimonSellick]

I'm building simple line charts in PHP and need to choose sensible y-axis limits for the data provided, probably along the lines of the 1, 2, 5 series. The x-axis values are just labels (e.g. month, week) and are provided for me.

The y-ordinates could be any value within sensible limits, for example 0.001 - 1000000. I'll ignore negative values for now, and I am interested only in a linear scale starting at 0. Although there is a wide possible range, any one graph would have all of its values within about an order of magnitude of each other.

I want a y-axis scale that puts the highest value reasonably near the top of the graph, and has sensible-looking maximum and intermediate grid labels.

For example, if the y values were

{0.03, 0.5, 1.1, 0.66}

..then I would want the y axis to be

{0, 0.2, 0.4, ..., 2.0}.

If they were

{7501, 8800, 6943, 1311}

..then I would want

{0, 1000, 2000, ..., 10000}.

I can see how to do this by (1) either analysing and altering a string conversion of the highest y-ordinate found, or by looping through a scaling routine; then (2) reconverting the massaged string to a number and performing a load of comparisons, but can anyone point me towards a more elegant solution?

[this seems to be a similar problem to

http://www.tek-tips.com/viewthread.cfm?qid=827174

which hasn't been answered]

Thanks in advance.
Simon.