HOUROFDAY must exist as a field in your data otherwise how will your expressions make any sense!? Your chart shows circles of different sizes for different hours of the day - that must come from somewhere! So all you need to do is find a way of limiting the displayed hours. Native QlikView functionality is to make a selection in this field, which you can do direct from the chart. If you want to use From and To variables then the Set Analysis expressions given above will achieve that.
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