.plot-container {
  position: relative;
}
.loading-spinner {
  position: absolute;
  left: 50%;
  top: 50%;
  z-index: -1;
  margin-top: -33px;  /* half of the spinner's height */
  margin-left: -33px; /* half of the spinner's width */
}
.plot.recalculating {
  z-index: -2;
}