Python Matplotlib X-axis label dual axis with dataframe

Question

I've got a dual axis bar and line plot using matplotlib. I read the data in as a dataframe,

[WEEK       SIGNUPS APPLICATIONS    PRECOURSE_WORK  QUALIFIED   ENROLLED    SPEND
 2019-10-07 5674    2938            2220            106         2           77581.67
 2019-10-14 4538    2225            2309            567         204         61258.08
 2019-10-21 3865    1997            1801            121         39          53700.58
 2019-10-28 3559    1886            1641            162         39          53543.28
 2019-11-04 3782    1946            1980            190         109         49495.64
 2019-11-11 4033    2035            1568            118         109         49952.17
 2019-11-18 3999    2009            1537            83          77          58545.72
 2019-11-25 6170    3322            1660            110         61          52332.4
 2019-12-02 5189    2658            7041            73          30          56727.55
 2019-12-09 4631    2497            7904            174         116         60977.49
 2019-12-16 4935    2501            3492            108         82          68179.54
 2019-12-23 5289    2603            1983            80          38          76956.81
 2019-12-30 5843    3037            2150            90          80          76246.14
 2020-01-06 4194    1930            1619            74          57          46114.68]

My code works and produces a graph (below)

Here is my code

import matplotlib.pyplot as plt
from pylab import rcParams
from matplotlib import style
style.use('seaborn-paper')
#print(plt.style.available)
rcParams['figure.figsize'] = 20, 10
#plt.xticks(df[['WEEK']])
ax = df[['SPEND']].plot(kind='bar', color = 'lightblue')
ax.set_ylabel("Spend",color="blue",fontsize=20)
ax.set_xlabel('Weeks',color="blue",fontsize=20)
ax2 = ax.twinx()
ax2.plot(df[['SIGNUPS','APPLICATIONS','ENROLLED']].values, linestyle='-', marker='o', linewidth=4.0)
fmt = '${x:,.0f}'
tick = mtick.StrMethodFormatter(fmt)
ax.yaxis.set_major_formatter(tick)

When I uncomment the line plt.xticks(df[['WEEK']]) I get the following error

ConversionError Failed to convert value(s) to axis unit.

Can anyone help me out?

Answer 1

plt.xticks is expecting the tick locations to be specified and optionally the labels, from the docs the signature is

xticks(ticks, [labels], **kwargs)

So when you do

plt.xticks(df[['WEEK']])

It is trying to interpret the dates in the 'WEEK' column as the locations for the ticks. What you want to do instead is use plt.set_xticklabels which expects only the labels be specified, ie

plt.set_xticklabels(df[['WEEK']])
# or
plt.set_xticklabels(df[['WEEK']].values)

Although you may also need to manually covert the values to strings, depending on how they are defined.