Every time time I look at a daily line chart I always seem to be alerted by the sudden drop off at the end of the chart. Then I realise it's just because the most recent day on the chart is today and the data isn't a complete day. So it's not really comparable with previous days.
Here are some ideas that might help:
  • Trend the line for today, so it continues from yesterday, with a dot dot dot
  • Never display today's data on a daily chart
  • Provide the option to remove/add today's data from the chart, perhaps "removed" is a default