this is the code in gnome-system-monitor that draws the curves

gistfile1.hpp

    cr = gdk_cairo_create (g->disp->window);

    cairo_set_line_width (cr, 1);
    cairo_set_line_cap (cr, CAIRO_LINE_CAP_ROUND);
    cairo_set_line_join (cr, CAIRO_LINE_JOIN_ROUND);
    cairo_rectangle (cr, g->rmargin + g->indent + FRAME_WIDTH + 1, FRAME_WIDTH - 1,
             g->draw_width - g->rmargin - g->indent - 1, g->real_draw_height + FRAME_WIDTH - 1);
    cairo_clip(cr);

    for (j = 0; j < g->n; ++j) {
        cairo_move_to (cr, x_offset, (1.0f - g->data[0][j]) * g->real_draw_height);
        gdk_cairo_set_source_color (cr, &(g->colors [j]));

        for (i = 1; i < LoadGraph::NUM_POINTS; ++i) {
            if (g->data[i][j] == -1.0f)
                continue;
            cairo_curve_to (cr, 
                       x_offset - ((i - 0.5f) * g->graph_delx),
                       (1.0f - g->data[i-1][j]) * g->real_draw_height + 3.5f,
                       x_offset - ((i - 0.5f) * g->graph_delx),
                       (1.0f - g->data[i][j]) * g->real_draw_height + 3.5f,
                       x_offset - (i * g->graph_delx),
                       (1.0f - g->data[i][j]) * g->real_draw_height + 3.5f);
        }
        cairo_stroke (cr);

    }