What are some good ways to compress data across time? - arrays

Question

I have an array of objects with time and value property. Looks something like this.
UPDATE: dataset with epoch times rather than time strings
[{datetime:1383661634, value: 43},{datetime:1383661856, value: 40}, {datetime:1383662133, value: 23}, {datetime:1383662944, value: 23}]
The array is far larger than this. Possibly a 6 digit length. I intend to build a graph to represent this array. Due to obvious reasons, I cannot use every bit of the data to build this graph (value vs time); so I need to normalize it across time.
So here's the main problem - There is no trend in the timestamp for these objects; so I need to dynamically choose slots of time in which I either average out the values or show counts of objects in that slot.
How can I calculate slots that user friendly. i.e per minute, hour, day, eight hours or so. I am looking at having a maximum of 25 slots done out of the array, which I show up on the graph.
I hope this helps get my point through.

Answer 1

You can convert the date/time into epoch and use numpy.histogram to get the ranges:
import random, numpy
l = [ random.randint(0, 1000) for x in range(1000) ]
num_items_bins, bin_ranges = numpy.histogram(l, 25)
print num_items_bins
print bin_ranges
Gives:
[34 38 42 41 43 50 34 29 37 46 31 47 43 29 30 42 38 52 42 44 42 42 51 34 39]
[ 1. 40.96 80.92 120.88 160.84 200.8 240.76 280.72
320.68 360.64 400.6 440.56 480.52 520.48 560.44 600.4
640.36 680.32 720.28 760.24 800.2 840.16 880.12 920.08
960.04 1000. ]

Answer 2

Hard to say without knowing the nature of your values, compressing values for display is a matter of what you can afford to discard and what you can't. Some ideas though:
histogram
candlestick chart

Answer 3

Is this JSON and the DateTimes transmitted as text?
Why not transmit the Date as a long (Int64), and use a method to convert to/from DateTime? Depending on which language you could use these implementations:
DateTime to Long in C#
Date to long using Unix timestamp in Java
That alone would save you a considerable amount of space, since strings are 16-bits per character and the long TimeStamp would be just 64 bits.