Matplotlib Plot points on an existing line, only by knowing x values

Question

I have 2 dataframes:

'stock' is a dataframe with columns Date and Price. 'events' is a dataframe with columns Date and Text.

My goal is to produce a graph of the stock prices and on the line place dots where the events occur. However, I do not know how to do 'y' value for the events dataframe as I want it to be where it is on the stock dataframe.

I am able to plot the first dataframe fine with:

plt.plot('Date', 'Price', data=stock)

And I try to plot the event dots with:

plt.scatter('created_at', ???, data=events)

However, it is the ??? that I don't know how to set

tdy · Accepted Answer · 2021-05-05 19:48:35Z

1

Assuming Date and created_at are datetime:

stock = pd.DataFrame({'Date':['2021-01-01','2021-02-01','2021-03-01','2021-04-01','2021-05-01'],'Price':[1,5,3,4,10]})
events = pd.DataFrame({'created_at':['2021-02-01','2021-03-01'],'description':['a','b']})

stock.Date = pd.to_datetime(stock.Date)
events.created_at = pd.to_datetime(events.created_at)

Filter stock by events.created_at (or merge) and plot them onto the same ax :

stock_events = stock[stock.Date.isin(events.created_at)]

# or merge on the date columns
# stock_events = stock.merge(events, left_on='Date', right_on='created_at')

ax = stock.plot(x='Date', y='Price')
stock_events.plot.scatter(ax=ax, x='Date', y='Price', label='Event', c='r', s=50)

edited May 5, 2021 at 19:48

answered May 5, 2021 at 18:14

tdy

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