Min Points to Create a Line Chart

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(This question has been seen in the interviews of the following companies: Google)
You task is to build a line chart for the server load change through time, given a sequence of load rate data. Each data point is a pair of numerics [int timestamp, double rate]. The timestamp starts from 0 and naturally increments by 1 per data point.
Find out the minimum number of points necessary for the line chart. Return a list of significant points in the chart...

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