How can I plot an average line for a scatter plot?

Question

I have the df below and I want to create 3 scatter plots (days vs A, days vs B, days vs C) displaying all data points plus a line that goes through the average for each day.

plt.plot(df.iloc[:,0],df.iloc[:,3], 'b') for example gives me a line that goes through each single point (3rd plot), but instead I want a line that goes through the average value.

Here is my code:

data=np.array([[1,4.4,40.1,55],
           [1,4.5,40.6,45.6],
           [1,4.4,41.5,61.3],
           [4,10,26,79.4],
           [4,11.2,25.3,80.9],
           [4,10.5,23.6,84],
           [10,5.6,12.7,58.2],
           [10,6,10.9,60.8],
           [10,7.3,8.7,70.5,],
           [15,2.5,5.4,98.7],
           [15,2.7,6.2,95.3],
           [15,2.8,4.7,88.9],
           [25,0.8,3.3,25.4],
           [25,0.5,1,28.6],
           [25,1,5,23.6]])

df = pd.DataFrame(data[:,0:],columns=['days','A','B','C'])

plt.subplot(3, 1, 1) 
plt.plot(df.iloc[:,0],df.iloc[:,1], 'ro')
plt.subplot(3, 1, 2) 
plt.plot(df.iloc[:,0],df.iloc[:,2], 'ro')
plt.subplot(3, 1, 3) 
plt.plot(df.iloc[:,0],df.iloc[:,3], 'ro')
plt.plot(df.iloc[:,0],df.iloc[:,3], 'b')

Answer 1

You can use groupby to group your values and then plot the average. The method to group and find the mean was adapted from this answer. The rest was the plotting part.

df = pd.DataFrame(data[:,0:],columns=['days','A','B','C'])
df_mean = df.groupby('days')['A','B','C'].mean()

plt.subplot(3, 1, 1) 
plt.plot(df.iloc[:,0],df.iloc[:,1], 'ro')
plt.plot(df_mean.index, df_mean['A'], '-r')

plt.subplot(3, 1, 2) 
plt.plot(df.iloc[:,0],df.iloc[:,2], 'ro')
plt.plot(df_mean.index, df_mean['B'], '-r')

plt.subplot(3, 1, 3) 
plt.plot(df.iloc[:,0],df.iloc[:,3], 'ro')
plt.plot(df_mean.index, df_mean['C'], '-r')

Answer 2

You should just create a vector with the mean value and plot it:

plt.plot(df.iloc[:,0], df.iloc[:,3].mean() + np.zeros_like(df.iloc[:,3]), 'g--')