// ...
// manually define X axis tick positions and labels
double[] xPositions = { 7, 21, 37, 46 };
string[] xLabels = { "VII", "XXI", "XXXVII", "XLVI" };
plt.XAxis.ManualTickPositions(xPositions, xLabels);

// manually define Y axis tick positions and labels
double[] yPositions = { -1, 0, .5, 1 };
string[] yLabels = { "bottom", "center", "half", "top" };
plt.YAxis.ManualTickPositions(yPositions, yLabels);
// ...