Added generic graph drawing loop; isolated point drawing of graph style 'Bar'

[wimrijnders]

Redo of PR #2205, which I messed up in a very bad way.

I hope this is good now.

All graph drawing routines follow the following structure:

Graph3d.prototype._redrawGraph = function() {
  var ctx = this._getContext()

  if (this.dataPoints === undefined || this.dataPoints.length <= 0)
    return; // TODO: throw exception?

  this._calcTranslations(this.dataPoints);

  for (i = 0; i < this.dataPoints.length; i++) {
    point = this.dataPoints[i];

    // Draw data point here
  }
};

It is possible to consolidate this loop and move graph-style specific point drawing
to separate routines. This PR shows how that will look like; I've done only a single
graph style (Bar) to illustrate this.

Changes:

• made generic routine _redrawDataGraph(), usable for all graph styles.
• made specific routine _redrawBarGraphPoint(), which draws a single point for style Bar
• as an extra, isolated the bar drawing code in _redrawBar(), so it can be reused.
• in method redraw(), pass _redrawBarGraphPoint as a reference to _redrawDataGraph()

The idea is:

• to make the drawing loop generic for all graph styles
• isolate the drawing of points for specific graph style in separate methods.
• do this for all present graph styles.

Notes

1. This is a very important update in regard to handling multiple graphs in a view.
The reason is that the elements of the multiple graphs will need to be drawn together at the same time, so that the overlap of the various elements are correct in the view.

2. I would much rather prefer to use graph-style specific classes here, instead of passing
a point-drawing method.
However, I fully appreciate that implementing that in one go will be a 'Big Bang' change
which is totally unoverseeable for reviewing. For this reason, I am offering the changes in smaller,
bite-size chunks. The classes will have to wait.

[wimrijnders]

The last commit adds a useful change with the determination of the point drawing method to use.

I added it because git refused me to make an empty commit.

[mojoaxel]

Keep up the work!

[mojoaxel]

@wimrijnders Have a look at git --amend.

[wimrijnders]

@mojoaxel thanks I will. Actually I'll do it now.

OK did some google research. I'm not sure how it would have helped me with my struggles in #2205 , but I have never used --amend so it was useful to learn.