Logarithmic scale is appropriate for most long-term predictions of curves that have some exponential growth. You can also use GGP to analyze stock values rather than indices. First of all, the shorter the period of time of your data, the higher the noise on them. For new companies, you usually don't have enough data. You should have at least a 5 year chart.
Noise always reduces the reliability of predictions in a very drastic way.
The full GGP version contains a feature for noise estimation, but up to now, I have not enough experience to estimate how much noise can be tolerated.