尝试从 pandas dataframe 获取 plot 数据

Question

I am trying to build a simple line-chart with a pandas dataframe. The dataframe contains a column with date entries. For each date there are several rows of data. I want to group the data by the date, count the entries per group and have a altair linechart present the data.

I've tried several approches but cannot find a solution. i think copying the data into a new dataframe would be the best solution. The Dataframe is populated from a csv by following function

def read_csv(rows):
    parse_dates = ['Gemeldet_Am']
    data = pd.read_csv(path+file,nrows=rows, parse_dates=parse_dates)
    data['Gemeldet_Am'] = pd.to_datetime(data['Gemeldet_Am']).dt.date
    return data

A print of the dataframe looks like this:

I want to group by the everything insinde the blue box and then count the data in the red box. The altair chart should then show for each day(blue data) the sum of the red data

Answer 1

Here is an example of a line chart with counts grouped by date with data that is similar to yours:

import altair as alt
import pandas as pd

df = pd.DataFrame({
    'date': pd.to_datetime(['2021-01-13', '2021-01-13', '2021-01-13', '2021-01-13',
                            '2021-01-14', '2021-01-14', '2021-01-14', '2021-01-15',
                            '2021-01-15', '2021-01-16', '2021-01-17', '2021-01-17']),
    'SAG': ['SAG-2101-1350', 'SAG-2101-1352', 'SAG-2101-1355', 'SAG-2101-1370',
            'SAG-2101-1373', 'SAG-2101-1378', 'SAG-2101-1382', 'SAG-2101-1385', 
            'SAG-2101-1391', 'SAG-2101-1393', 'SAG-2101-1394', 'SAG-2101-1397']
})

alt.Chart(df).mark_line().encode(
    x='date:T',
    y='count():Q'
)