Hello,
you need to use an XY scatter chart, since line charts don't show units smaller than a day on the x axis.
Format the X axis and set the major unit to 0.010417, which is the numerical equivalent of 15 minutes. Note that with 15 minute increments, you need to make the chart about three screens wide, so all data labels show. You also may want to turn the axis labels 90 degrees.
You can set the number format of the axis to "Linked to source" so it will show the same format as your source data.