Improve low number of fixation visualization

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When the number of fixation per minute is too low (below 50 fixations), the NNI value is not reliable.
In the current version of ASTEF, during navigation through minutes with the arrow, a red dot is shown on the graph to show that fixation number is too low. However, some kind of indication should be always visible. Moreover, when exporting timeseries, not reliable value should be marked.