Highlight points in Matplotlib line plot with index as datetime

Question

I am trying to mark data points in the line plot using markevery option in matplotlib. I am plotting data with index in datetime format. When I create a list for the markevery I get this error

markevery=['13:15:00', '13:30:00', '13:45:00', '14:00:00', '14:15:00', '14:30:00'] is iterable but not a valid numpy fancy index

My code

markers_on= ['13:15:00','13:30:00','13:45:00','14:00:00','14:15:00','14:30:00']

ax1 = plt.subplot()

ax1.plot(data,'-gD', markevery=markers_on)

ax1.set_xlabel("Time (HH:MM:SS)",fontsize=15)
ax1.set_ylabel("Temp",fontsize=15)

My data looks like this:

time	temp
09:59:45	0.011154
10:00:00	0.028380
10:00:15	0.072002
10:00:30	0.151146
10:00:45	0.200887

How can I use list of datetime index to mark on the plot?

I have tried to convert the values in the list to datetime objects, but did not work.

Answer 1

You could try to use pandas.Index.get_indexer to convert your list of strings markers_on to a list of positions where the index matches the target values:

import matplotlib.pyplot as plt
import pandas as pd

# make data
d = {'time': ['09:59:45','10:00:00','10:00:15','10:00:30','10:00:45'],
    'temp': [0.011154, 0.028380, 0.072002, 0.151146, 0.200887]}

# make a data frame
data = pd.DataFrame(d, index=d['time'])

# a couple of timestamp strings to set markers at 
markers_on = ['10:00:15', '10:00:45']

# plot data, showing markers only for the specified timestamp strings
ax = plt.subplot()
ax.plot(data['time'], data['temp'], '-gD', markevery=data.index.get_indexer(markers_on))
ax.set_xlabel('Time (HH:MM:SS)', fontsize=15)
ax.set_ylabel('Temp', fontsize=15)
plt.show()

The result: