| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

Last 100 entries

Real-life Financial Co Without ACID Database...; Flexible Muscle-Based Locomotion for Bipedal Creatures; SQL Performance Explained; The Little Manual of API Design; Multiple Word Sizes; CRC - Next Steps; FizzBuzz; Update on CRCs; Decent Links / Discussion Community; Automated Reasoning About LLVM Optimizations and Undefined Behavior; A Painless Guide To CRC Error Detection Algorithms; Tests in Julia; Dave Eggers: what's so funny about peace, love and Starship?; Cello - High Level C Programming; autoreconf needs tar; Will Self Goes To Heathrow; Top 5 BioInformatics Papers; Vasovagal Response; Good Food in Vina; Chilean Drug Criminals Use Subsitution Cipher; Adrenaline; Stiglitz on the Impact of Technology; Why Not; How I Am 5; Lenovo X240 OpenSuse 13.1; NSA and GCHQ - Psychological Trolls; Finite Fields in Julia (Defining Your Own Number Type); Julian Assange; Starting Qemu on OpenSuse; Noisy GAs/TMs; Venezuela; Reinstalling GRUB with EFI; Instructions For Disabling KDE Indexing; Evolving Speakers; Changing Salt Size in Simple Crypt 3.0.0; Logarithmic Map (Moved); More Info; Words Found in Voynich Manuscript; An Inventory Of 3D Space-Filling Curves; Foxes Using Magnetic Fields To Hunt; 5 Rounds RC5 No Rotation; JP Morgan and Madoff; Ori - Secure, Distributed File System; Physical Unclonable Functions (PUFs); Prejudice on Reddit; Recursion OK; Optimizing Julia Code; Cash Handouts in Brazil; Couple Nice Music Videos; It Also Works!; Adaptive Plaintext; It Works!; RC5 Without Rotation (2); 8 Years...; Attack Against Encrypted Linux Disks; Pushing Back On NSA At IETF; Summary of Experimental Ethics; Very Good Talk On Security, Snowden; Locusts are Grasshoppers!; Vagrant (OpenSuse and IDEs); Interesting Take On Mandela's Context; Haskell Cabal O(n^2) / O(n) Fix; How I Am 4; Chilean Charity Supporting Women; Doing SSH right; Festival of Urban Intervention; Neat Idea - Wormholes Provide Entanglement; And a Link....; Simple Encryption for Python 2.7; OpenSuse 13.1 Is Better!; Little Gain...; More Details on Technofull Damage; Palmrest Cracked Too....; Tecnofull (Lenovo Support) Is Fucking Useless; The Neuroscientist Who Discovered He Was a Psychopath; Interpolating Polynomials; Bottlehead Crack as Pre-amp; Ooops K702!; Bottlehead Crack, AKG K701; Breaking RC5 Without Rotation; Great post thank you; Big Balls of Mud; Phabricator - Tools for working together; Amazing Julia RC5 Code Parameterized By Word Size; Chi-Square Can Be Two-Sided; Why Do Brits Accept Surveillance?; Statistics Done Wrong; Mesas Trape from Bravo; European Report on Crypto Primitives and Protocols; Interesting Omissions; Oryx And Crake (Margaret Atwood); Music and Theory; My Arduino Programs; Elliptic Curve Crypto; Re: Licensing Interpreted Code; Licensing Interpreted Code; ASUS 1015E-DS03 OpenSuse 12.3 SSD; translating lettuce feature files into stub steps files; Re: translating lettuce feature files into stub steps files; A Tale of Two Psychiatrists; The Real Reason the Poor Go Without Bank Accounts

© 2006-2013 Andrew Cooke (site) / post authors (content).

Auto-Scaling Date Axes in Python

From: andrew cooke <andrew@...>

Date: Wed, 28 Jul 2010 10:16:53 -0400

There's a nice algorithm for auto-scaling axes, called the "nice number
algorithm", written by Paul Heckbert and published in "Graphics Gems" -
http://books.google.com/books?id=fvA7zLEFWZgC&pg=PA61&lpg=PA61&dq=nice+numbers+graphics+gems&source=bl&ots=7LdCq3nI-j&sig=L8qoZ8l_a95KAtHmMjagJ8cC0U0&hl=en&ei=KDhQTKLwGcT48AbTsvnEAQ&sa=X&oi=book_result&ct=result&resnum=2&ved=0CBYQ6AEwAQ#v=onepage&q&f=false

The routines below implement this, but are parameterised over the number base
used, so can also be used for axes based on units that repeat over multiples
of 12, 60, or any other value.


from calendar import timegm
from math import floor, log, log10, ceil
from time import gmtime

# These allow the use with base 10, 12 and 60:
LIM10 = (10, [(1.5, 1), (3, 2), (7, 5)], [1, 2, 5])
LIM12 = (12, [(1.5, 1), (3, 2), (8, 6)], [1, 2, 6])
LIM60 = (60, [(1.5, 1), (20, 15), (40, 30)], [1, 15, 40])

def heckbert_d(lo, hi, ntick=5, limits=None):
    '''
    Calculate the step size.
    '''
    if limits is None:
        limits = LIM10
    (base, rfs, fs) = limits
    def nicenum(x, round):
        step = base ** floor(log(x)/log(base))
        f = float(x) / step
        nf = base
        if round:
            for (a, b) in rfs:
                if f < a:
                    nf = b
                    break
        else:
            for a in fs:
                if f <= a:
                    nf = a
                    break
        return nf * step
    delta = nicenum(hi-lo, False)
    return nicenum(delta / (ntick-1), True)

def heckbert(lo, hi, ntick=5, limits=None):
    '''
    Calculate the axes lables.
    '''
    def _heckbert():
        d = heckbert_d(lo, hi, ntick=ntick, limits=limits)
        graphlo = floor(lo / d) * d
        graphhi = ceil(hi / d) * d
        fmt = '%' + '.%df' %  max(-floor(log10(d)), 0)
        value = graphlo
        while value < graphhi + 0.5*d:
            yield fmt % value
            value += d
    return list(_heckbert())


This can then be used with a range of seconds as follows:


def autoscale_time(start, end):
    '''
    Yields a sequence of epochs that are nicely spaced.

    start and end are Unix epochs.
    '''
    time_chunks = [('days', 3 * 24 * 60 * 60, 24 * 60 * 60, 2, None),
                   ('hours', 3 * 60 * 60, 60 * 60, 3, LIM12),
                   ('minutes', 3 * 60, 60, 4, LIM60),
                   ('seconds', 0, 1, 5, LIM60)]
    for (name, limit, secs, sindex, limits) in self.time_chunks:
	if (end - start) > limit:
	    break
    d = heckbert_d(start / secs, end / secs, limits=limits)

    # zero out lower steps, so that we get a starting date that's an
    # integral number of units
    stime = list(gmtime(start))
    for i in range(sindex, 9):
	stime[i] = 0

    # generate a sequence of epochs (cannot use the usual heckbert routine 
    # because formatting will be different)
    value = timegm(stime)
    while value <= end:
	if value >= start:
            yield value
	value += d * secs


This could be extended further by:

- having different formats in the time_chunks parameter, so that different
  intervals are formatted differently

- adding months etc.  This would require changing the "secs" increment to be a
  timedelta and working with datetime instances rather than epochs (because
  months are not all equally sized).


NOTE: The code above is cut + pasted from some working code and is not tested
in its existing form; I may have introduced a bug somewhere, but hopefully
this illustrates the idea.

Andrew

Comment on this post