Using matplots 'axis' command makes my plot disappear

Question

I have a list of prices that I want to plot using matplotlib, and if I just do so, everything is fine. Here is my code:

def plot_prices_and_events(prices, buy=None, sell=None):
    data = []
    max_price = 0
    min_price = sys.maxint
    values = prices.get_values()
    for price in values:
        current_price = price[1]
        data.append(current_price)
        if current_price > max_price:
            max_price = price[1]
        if current_price < min_price:
            min_price = current_price
    plt.ylabel('Bitcoin price in EUR')
    plt.xlabel('Date')
    start_date = datetime.datetime.fromtimestamp(values[0][0])
    end_date = datetime.datetime.fromtimestamp(values[-1][0])
    plt.plot(data)
    #plt.axis([start_date, end_date, min_price, max_price])
    if buy is not None:
        plt.plot(buy)
    if sell is not None:
        plt.plot(sell)
    plt.show()

With the one line commented out, everything is fine:

Yet, if I try to set the scale for the x-axis, thus uncommenting the line, the axis are displayed correctly, but my plot disappears:

This is reproducible on two machines and by commenting and uncommenting the corresponding line. Am I missing something? I have already played around with the order of the commands, I checked the official tutorials and did some googling. The issue does not seem to be too far spread, so I hope for you people.

Thanks in advance and I hope I'm just stupid. ^^

EDIT: The issue is solved, but the solution is partly to be found in the comments. Bottom Line: The prices were not correctly ordered.

Obviously your plt.plot() plots data as a function from [0:number_of_times] to prices. You have to specify the X-range correctly also in your plt.plot() — Dima Chubarov
– Dima Chubarov, Commented Jun 7, 2017 at 9:38

acdr · Accepted Answer · 2017-06-07 09:49:38Z

2

As @Dmitri Chubarov said in a comment, by calling plt.plot(data), you are only providing vertical co-ordinates. matplotlib then has to assume that the horizontal co-ordinates are just [0:len(data)].

You later tell it to move the horizontal axis to [start_date, end_date]. However, none of your data points have a horizontal co-ordinate in this range, because the horizontal co-ordinates are [0:len(data)].

You'd have to provide horizontal co-ordinates to your data as well to make this work better. E.g. plt.plot(dates, data), where dates is a list of dates of equal length as data.

answered Jun 7, 2017 at 9:49

acdr

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3 Comments

Aarkon Over a year ago

Thank you both. This seems obvious once you know about it. I've added a list of dates for every price and added it to the plot()-call. Now the axis is labeled correctly, but my plot goes nuts: i.imgur.com/gusg9yD.png

acdr Over a year ago

Are you sure your data are sorted by their date?

Aarkon Over a year ago

I had not doubt they were until now, but I'll have a closer look into that before going any further. Thank you!

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