Why does my line chart only fill half the space?

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I am guessing that you included several blank rows, or that the two data sets use different numbers of rows, or something like that. The blank space is for "blank" categories in the chart's source data. We would probably need to see the chart in its context in the spreadsheet to say more.

Is there a reason you are using a line chart? My inclination for something like this would be to use an XY scatter chart (assuming the times are real time values and not text strings). An XY scatter chart can treat those times as real time values and not arbitrary "categories", so that data will space according to the actual time stamp, which may be a more accurate representation of the data.

Is there a reason you cannot combine the date information with the time information? Even if it isn't the actual calendar date, but adding a "day 1" (1+time stamp), "day 2" (2+time stamp), "day 3" (3+timestamp), etc. information to the time stamp would be enough for Excel to put the data in correct sequential order. Can I assume you are familiar with how Excel stores and manipulates date and time information (http://www.cpearson.com/Excel/datetime.htm )?

Thanks, unfortunately, my workbook is too large to attach

The actual workbook is not necessary, just a representative sample so that the issue can be seen in context.