Question

Wow, this one is though!

I’m trying to find in PHP the % of a value relative to the Y axis. If we refer to this graph : http://en.wikipedia.org/wiki/Semi-log_graph (2009 outbreak of influenza A), let’s say that I want to find what % is a value “256” on the chart.

Visually, it’s easy : it’s a bit more than a 1/3 or 33%. If we look at the 1024 value, it’s around 50% of the height of the Y axis. 131072 would be 100%.

So how do I calculate this with PHP?

Let’s take this graph and take X = day 0 and Y = 256. What is 256 as a % of Y ?

Thanks a lot to anyone can compute this baby 🙂

Answer 1

percent = 100 * ( log(y) - log(y1) ) / ( log(y2) - log(y1) )

where

y = value 
y1 = smallest value in y-axis
y2 = largest value in y-axis.

when y1 = 1.0 then you can simplify the other answers given here (since log(1)=0 by definition)

percent = 100 * log(y)/log(y2)

Note that not all log charts have 1.0 as the lowest value.