how to add a string to a plot annotation in python?

Im setting up a cross sectional plot which illustrates dischargements at verticals; I'd like to annotate both the discharge and the unit, but I dont know how to add the unit to the plot inside of the zip-function.

more than happy about hints and suggestions :)

the input file looks as follows:

distance [m]    depth [m]   Q [l/s]
0   0.1     0.236   3.172
1   0.6     0.302   10.156
2   1.1     0.324   26.020
3   1.6     0.388   35.320
4   2.1     0.400   36.760
5   2.6     0.364   40.420
6   3.1     0.394   46.240
7   3.6     0.430   49.460
8   4.1     0.410   47.680
9   4.6     0.386   48.880
10  5.1     0.360   33.640
11  5.5     0.110   3.662

thats my current state:

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

df=pd.read_csv(r'test.csv',sep=';')

#plot
x= df['distance [m]']
y= df['depth [m]']
Q= round(df['Q [l/s]'],2)
plt.gca().invert_yaxis()
plt.fill_between(x,y,0, color='skyblue')
#plt.plot(x,y,color='blue')
plt.xticks(np.arange(0, max(x)+0.5, 0.5))
plt.yticks(np.arange(round(min(y)), max(y)+0.1, 0.05))
plt.ylim()

ylab='waterdepth [m]'
xlab='distance to left river bank [m]'
title='discharge at river chainage X'
plt.ylabel(ylab)
plt.xlabel(xlab)
plt.title(title,weight='bold')

for xcoo,ycoo in zip(x,y):
    plt.vlines(x=xcoo,ymin=0,ymax=ycoo,lw = 1,color='0.5',linestyles='dashed')
    
    
for xcoo,ycoo,q_test in zip(x,y,Q):
    plt.annotate(q_test,xy=(xcoo,(ycoo/2)),color='red',weight='bold',ha='center',fontsize=7)
    

plt.annotate(q_test+' l/s', xy=..., ...) results in:

TypeError                                 Traceback (most recent call last)
<ipython-input-59-bc9e0de80690> in <module>
     22 
     23 for xcoo,ycoo,q_test in zip(x,y,Q):
---> 24     plt.annotate(q_test+'l/s',xy=(xcoo,(ycoo/2)),color='red',weight='bold',ha='center',fontsize=7)
     25 
     26 #plt.savefig('test_plot.png',bbox_inches="tight", dpi = (300))

TypeError: unsupported operand type(s) for +: 'float' and 'str'