Provide more details in record line chart tooltip.

[hawjo01]

Display all values as integers and group data by time in case of multiple entries per day.

[github-actions]

LCOV of commit 05f4280 during Analyze and Test on Pull Request #36

Summary coverage rate:
  lines......: 54.8% (246 of 449 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate:
                                          |Lines       |Functions  |Branches    
  Filename                                |Rate     Num|Rate    Num|Rate     Num
  ==============================================================================
  lib/utils/datetime_utils.dart           | 0.0%      4|    -     0|    -      0
  lib/widgets/records_line_chart.dart     | 0.0%     63|    -     0|    -      0