Double_t fMaximum Maximum value for plotting along y
Double_t fMinimum Minimum value for plotting along y
Maybe I will shed some light:
You see, those minimum and maximum values in histograms are more of user set limits, that are SUPPOSED to be used for drawing only, and also for computing integrals, finding local minimums and such on that part of histogram that is drawn right now. They stopped being “real” minimums a while ago, when people wanted to be able to put left and right limits on a histogram, draw it, and compute integral within the limits that are visible only. Graphs use histogram for drawing, and they themselves have fMinimum and fMaximum for plotting only, double trouble now 
. TGraph::GetMinimum() returning -1111 means that no user set limit exists in TGraph. Interesting, huh ? Underlying histogram computes limits for itself within its binning precision because it has to accommodate the whole graph in absence of user limits.
I don’t know how to best remedy this confusion, maybe add methods GetAbsoluteMinimumX-Y-or-Z-() which always returns what people think minimum means irrespectively of user set drawing limits. Or, add a whole spectra of methods interfacing user to all possible operations on arrays.
Anyhow, without changing anything - here is a simple recipe: if you want to do something with an array, you do something with the array, instead of jamming it into TGraph and then asking TGraph to interface you with all kinds of mathematical operations on the array, that you might come up with. Savvy ?