Using fixed-size data buffer with GtkPlot

I'm just starting with GtkExtra, and, probably like everybody else on this
list, having troubles learning
the API.  I would like to be able to append points and plot them one at a
time, which is pretty much
what the 'testrealtime' example does.  The additional constraint I have is
that, unlike 'testrealtime', I
cannot store data points in a dynamically allocated buffer and let this
buffer grow indefinitely.

Here is the 'update' function I call to add a single point to the plot,
while using two C-style arrays:

#define NPOINTS 5000
gint runningCount;
gdouble px[NPOINTS];
gdouble py[NPOINTS];

gboolean update(gpointer)
{
  gdouble x,y;
  getPoint(runningCount,x,y);

  gdouble xmin, xmax; /* [xmin,xmax) */
  gtk_plot_get_xrange(plot, &xmin, &xmax);
  if (runningCount >= NPOINTS) {
    runningCount = 0;

    px[0] = x; py[0] = y;
    gtk_plot_set_xrange(plot, xmin + 2*M_PI, xmax + 2*M_PI);
    gtk_plot_data_set_numpoints(dataset, 1);
    gtk_plot_data_dimension_set_points(dataset, "x", px);
    gtk_plot_data_dimension_set_points(dataset, "y", py);

    gtk_plot_data_draw_points(dataset, 1);
    gtk_plot_canvas_paint(GTK_PLOT_CANVAS(canvas));

    gtk_widget_queue_draw(GTK_WIDGET(canvas));
  }
  else {
    px[runningCount] = x; py[runningCount] = y;
    gtk_plot_data_set_numpoints(dataset, dataset->num_points+1);

    gtk_plot_data_draw_points(dataset, 1);
    gtk_plot_refresh(plot, NULL);
  }

  ++runningCount;

  return TRUE;
}

Unfortunately, I only get meaningful results until the predicate in the
if-statement
becomes true.  After that, I don't see anything being plotted at all.  Is
there any
way to force the widget to "forget" the previous data I fed to it and start
from
scratch?  Once again, my goal is to use a fixed-size array to store data
points,
so I wouldn't have to worry about allocating memory on demand.


Many thanks,

Nickolai