[apps/graph] GraphView: change the step with which the function is evaluated

to draw curve.
Fix bug: polar function sin(5θ) was not drawn at all

apps/graph/graph/graph_view.cpp

@@ -31,7 +31,18 @@ void GraphView::drawRect(KDContext * ctx, KDRect rect) const {
     Shared::CartesianFunction::PlotType type = f->plotType();
     float tmin = f->tMin();
     float tmax = f->tMax();
-    float tstep = (tmax-tmin)/10.0f;
+    /* The step is a fraction of tmax-tmin. We will evaluate the function at
+     * every step and if the consecutive dots are close enough, we won't
+     * evaluate any more dot within the step. We pick a very strange fraction
+     * denominator to avoid evaluating a periodic function periodically. For
+     * example, if tstep was (tmax - tmin)/10, the polar function r(θ) = sin(5θ)
+     * defined on 0..2π would be evaluated on r(0) = 0, r(π/5) = 0, r(2*π/5) = 0
+     * which would lead to no curve at all. With 10.0938275501223, the
+     * problematic functions are the functions whose period is proportionned to
+     * 10.0938275501223 which are hopefully rare enough.
+     * TODO: The drawCurve algorithm should use the derivative function to know
+     * how fast the function moves... */
+    float tstep = (tmax-tmin)/10.0938275501223f;
 
     // Cartesian
     if (type == Shared::CartesianFunction::PlotType::Cartesian) {