private double getTimeY(long t0) {
   double y0 = t0 - history_graph.getStartTime();
   y0 =
       y0
               / (history_graph.getEndTime() - history_graph.getStartTime() + 1)
               * (time_end - time_start)
           + time_start;
   return y0;
 }
  /** ***************************************************************************** */
  private BddtHistoryItem getItemAtPoint(int x, int y) {
    if (object_width == 0) return null;

    double t =
        history_graph.getStartTime()
            + (y - time_start)
                / (time_end - time_start)
                * (history_graph.getEndTime() - history_graph.getStartTime() + 1);
    if (t < history_graph.getStartTime() || t > history_graph.getEndTime()) return null;

    GraphObject gobj = getObjectAtPoint(x, y);

    // correlate with blocks
    if (gobj != null) {
      GraphBlock gb0 = null;
      double dtime = 0;
      for (GraphBlock gb : gobj.getBlocks()) {
        double dt = gb.getEndTime() - gb.getStartTime();
        if (t >= gb.getStartTime() && t <= gb.getEndTime()) {
          if (gb0 == null || dt < dtime) {
            gb0 = gb;
            dtime = dt;
          }
        }
      }
      if (gb0 != null) {
        BddtHistoryItem bi0 = null;
        dtime = 0;
        for (BddtHistoryItem bhi : gb0.getItems()) {
          double dt = t - bhi.getTime();
          if (dt >= 0) {
            if (bi0 == null || dt < dtime) {
              bi0 = bhi;
              dtime = dt;
            }
          }
        }
        if (bi0 != null) return bi0;
      }
    }

    // correlate with links
    double delta =
        (history_graph.getEndTime() - history_graph.getStartTime()) / (time_end - time_start) * 3;

    double dtime = 0;
    GraphLink gl0 = null;
    for (int i = 0; i < history_graph.getNumObjects(); ++i) {
      GraphObject go = history_graph.getObject(i);
      double x0 = graph_locations.get(go);
      for (GraphLink gl : go.getLinks()) {
        double x1 = graph_locations.get(gl.getToObject());
        if (x > x0 && x < x1) {
          double dt = Math.abs(t - gl.getTime());
          if (gl0 == null || dt < dtime) {
            gl0 = gl;
            dtime = dt;
          }
        }
      }
    }
    if (gl0 != null && dtime <= delta) {
      return gl0.getItem();
    }

    return null;
  }