The guided X,Y lines are called "grid lines" in ChartDirector documentation.The grid line colors can be set in XYChart.setPlotArea. You can draw them as dashed lines by using a "dash line color" as the line color.
c->setPlotArea(50, 55, 400, 400, Chart::Transparent, -1, -1,
c->dashLineColor(0xaa000000, Chart::DotLine), // horizontal grid line as dash line
0xcccccc); // vertical grid line as solid line
The following is an example that uses dash lines for both the x and y grid lines.