Fix fitting with gaus(0)/gaus and properly make use of GetParValue(n) instead of fParams[n] (fixes differences between ROOT 5 & ROOT 6)

Author: bellenot

scripts/JSRootCore.js

@@ -1170,6 +1170,7 @@
             if (! ('_func' in this) || (this._title !== this.fTitle)) {
 
               var _func = this.fTitle;
+              if (_func === "gaus") _func = "gaus(0)";
 
               if ('formulas' in this)
                  for (var i=0;i<this.formulas.length;++i)

scripts/JSRootMath.js

@@ -573,23 +573,23 @@
 
    JSROOT.Math.gaus = function(f, x, i) {
       // function used when gaus(0) used in the TFormula
-      return f.fParams[i+0] * Math.exp(-0.5 * Math.pow((x-f.fParams[i+1]) / f.fParams[i+2], 2));
+      return f.GetParValue(i+0) * Math.exp(-0.5 * Math.pow((x-f.GetParValue(i+1)) / f.GetParValue(i+2), 2));
    };
 
    JSROOT.Math.gausn = function(f, x, i) {
-      return JSROOT.Math.gaus(f, x, i)/(Math.sqrt(2 * Math.PI) * f.fParams[i+2]);
+      return JSROOT.Math.gaus(f, x, i)/(Math.sqrt(2 * Math.PI) * f.GetParValue(i+2));
    };
 
    JSROOT.Math.expo = function(f, x, i) {
-      return Math.exp(f.fParams[i+0] + f.fParams[i+1] * x);
+      return Math.exp(f.GetParValue(i+0) + f.GetParValue(i+1) * x);
    };
 
    JSROOT.Math.landau = function(f, x, i) {
-      return JSROOT.Math.Landau(x, f.fParams[i+1],f.fParams[i+2], false);
+      return JSROOT.Math.Landau(x, f.GetParValue(i+1),f.GetParValue(i+2), false);
    }
 
    JSROOT.Math.landaun = function(f, x, i) {
-      return JSROOT.Math.Landau(x, f['fParams'][i+1],f['fParams'][i+2], true);
+      return JSROOT.Math.Landau(x, f.GetParValue(i+1),f.GetParValue(i+2), true);
    }
 
    return JSROOT;