Yeah I know exactly what you mean. Not sure how to fix though, maybe in the UI could show something like "New" but I presume the API would maybe still expect some number or so.

I thought about using ∞ (∞) instead of 100%, but that is also not a valid number, so duh.

I thought about this some more.

Instead of changing the output, it might be better to change the input?

Here it is hard coded to return 100% if the old value (divisor) is 0. Well we could instead provide an arbitrary divisor for the calculation.

I think there are 2 possible arbitrary numbers to choose from:

return Piwik::getQuotientSafe($dividend, 1, $this->quotientPrecision + 2);

Will return the same as if the old value was 1.

return Piwik::getQuotientSafe($dividend, 1.0E-10, $this->quotientPrecision + 2);

Will return a huge number and is easily distinguishable from the old value 1.

What are your thoughts with this? @tsteur

Not really sure what is best here. Maybe @mattab has some thoughts.

This might help:
https://stackoverflow.com/questions/19908431/how-to-calculate-percentage-when-old-value-is-zero

Anyway, once I spoke with someone who was in financial business / calculations and told me that if initial value is 0, than whatever next positive value comes up (e.g. next month) it's assumed to be 100%. Because, strictly mathematically speaking, it would be +inf (division by 0). I think it is just an assumption / by convention, depending on department, field etc. By that logic, if you dropped from a positive value to 0, it should be -100% by that very same convention. And, if you think about it, then it makes sense.