When you choose to import a text file into LabChart, it is necessary to have either the times in the first column of the text or you will need to input the sampling rate at which the data was captured.

In order to avoid this, you will need to ensure that the data points are evenly spaced in the text file prior to import into LabChart.
If any sample points are missing or the timing in the text column is inconsistant, LabChart will insert a block marker to show that the data is not continuous. A block marker is a vertical blue line through the data which occurs whenever you start and stop recording in LabChart, again to show the data is not continuous.

In order to avoid this, you will need to ensure that the data points are evenly spaced in the text file prior to import into LabChart.