How to interpret MM_DD as time information for Altair graph?

Question

I am struggling to interpret a month and day (MMDD, or MM_DD in my case) info as "Time" in my Altair graph.

The data I am working with contains the daily level of a lake from year 1850 until today. I calculated the min, max, mean value for every individual day of the year (mm_dd). Then, I converted my data frame to a long format, which now looks like this:

[]	mm_dd	value_type	levelcm	month
0	01_01	value_min	242	01
1	01_02	value_min	243	01
2	01_01	value_mean	301	01
3	01_02	value_mean	300	01
4	01_01	value_max	380	01
5	01_02	value_max	380	01

With

    alt.Chart(df_mmm_long).mark_line().encode(
      x='mm_dd',
      y='levelcm',
      color='value_type'
    )

I am getting

altair_mm_dd

Ideally, I would like the line chart to be based on the mm_dd information, but have a monthly scale on the X-axis.

user_2186559 · Answer

If you change a type of the column with dates to datetime, Altair will nicely group labels for you. Here in my example I took Rhein data for the last 25 years or so... :

a=df_.groupby(["Month", "Tag"], as_index="False").agg(minimum = ('Level', 'min'), \
                                 average = ('Level', 'mean'),
                                 maximum = ('Level', 'max')).reset_index()#.transpose()


a.Month=a.Month.replace({'Januar':1, 'Februar':2, 'Maerz':3, 'April':4, 'Mai':5, 'Juni':6, 'Juli':7,
       'August':8, 'September':9, 'Oktober':10, 'November':11, 'Dezember':12})

#combine day and month to date column
a['date']=a.Tag.astype(str)+"/"+a.Month.astype(str)+"/2020"
a.date=pd.to_datetime(a['date'], errors='ignore')


alt.Chart(a).transform_fold(
['minimum', 'average', 'maximum']).mark_line().encode(
x=alt.X("date:T"), 
y=alt.Y("value:Q",),
color='key:N'
).properties(width=600)

still not the nicest chart and requires some work, but as a start)

I guess you will add a curent level for 2022, so it's fine if the column date is 2022. In your example - they also have a current year at the axes labels

link to the data - Rhein data if someone wants to help P.S. Or you want a chart by month like that one (source:datawrapper)

How to interpret MM_DD as time information for Altair graph?

1 Answers