message: Show span time when selecting area on graph
diff: @@ -1559,9 +1559,9 @@ void TraceWin::render_mouse_graph( class graph_info_t *pgi )
  if ( ImGui::IsMouseDown( 0 ) )
  {
  std::string time_buf0 = ts_to_timestr( event_ts0, m_tsoffset );
  - std::string time_buf1 = ts_to_timestr( event_ts1, m_tsoffset );
  + std::string time_buf1 = ts_to_timestr( event_ts1 - event_ts0 );
  - ImGui::SetTooltip( "%s..%s", time_buf0.c_str(), time_buf1.c_str() );
  + ImGui::SetTooltip( "%s (%s ms)", time_buf0.c_str(), time_buf1.c_str() );
  }
  else
  {
  