remove incorrect gaps caused by the end time of trace at the end of data point at position number of sample -1 in low sample rate data

Closes #229 (closed) This one along with !245 (merged) closes #154 (closed)

The problem caused from changing definition of end of a record frame in !245 (merged).

If the added trace is one sample apart from the previous one, gap won't be added and the last point of the previous one will be remove to keep the plotting sample rate consistence for the view