color and size variation for single line - Altair

Question

I'm trying to change color and size of a line, basing on attributes variations. I tried with trail because it's the only example i found in the doc.
My test:

datas = pd.DataFrame({'num': range(10),
                      'value': [5, 5, 10, 10, 3, 3, 9, 9, 7, 7],
                      'size': [0.5, 0.5, 1, 1, 0.3, 0.3, 0.9, 0.9, 0.7, 0.7],
                      'otherMarker': ['foo']*2 + ['bar']*2 + ['foo']*2 + ['bar']*2 + ['egg']*2}
alt.Chart(datas).mark_trail.encode(
    x='num',
    y='value',
    size='size:Q',
    color='otherMarker:N')

gives me

instead i'd like

.
How can i achieve this ?

Answer 1

Using impute=alt.ImputeParams(value=None) will help you avoid that joining. You can also look at the detail parameter for this.

plt =alt.Chart(datas).mark_trail().encode(
    x='num',
    y=alt.Y('value', 
            impute=alt.ImputeParams(value=None)
        ),
    size='size:Q',
    color='otherMarker:N'
)
plt

A brute force approach to join the lines like the one shown in your image will be

# generate a dataframe for the lines connecting different 'otherMarker' types
def get_joining_lines(df):
    ids = datas[(datas['otherMarker']!=datas['otherMarker'].shift(-1))].index.to_list()[0:-1] # remove the last index
    ar = []
    for i in ids:
        temp = datas.loc[[i, i+1]] #adjacent indexes where the change happens
        temp['otherMarker'] = datas.loc[i]['otherMarker'] # change the tag of the last tag to be equal the previous
        ar.append(temp)
    df_temp = pd.concat(ar)
    return(df_temp)

df_temp = get_joining_lines(datas)
edge = alt.Chart(df_temp).mark_trail().encode(
    x='num',
    y=alt.Y('value', impute=alt.ImputeParams(value=None)),
    color='otherMarker:N'
)
edge

The color mapping is different from the initial plot, but this will get resolved when you combine them. If you want, you can hard code the color mapping using range and domain in the scale argument while defining color.

Finally, you can layer these two together to get

edge+ plt