Why is plot returning "ValueError: could not convert string to float:" when a dataframe column of floats is being passed to the plot function? - python-3.x

Question

I am trying to plot a dataframe I have created from an excel spreadsheet using either matplotlib or matplotlib and pandas ie. df.plot. However, python keeps returning a cannot convert string to float error. This is confusing since when I print the column of the dataframe it appears to be all float values.
I've tried printing the values of the dataframe column and using the pandas.plot syntax. I've also tried saving the column to a new variable.
import pandas as pd
from matplotlib import pyplot as plt
import glob
import openpyxl
import math
from openpyxl.utils.dataframe import dataframe_to_rows
from openpyxl.styles import Border, Side, Alignment
import seaborn as sns
import itertools
directory = 'E:\some directory'
#QA_directory = directory + '**/*COPY.xlsx'
wb = openpyxl.load_workbook(directory + '\\Calcs\\' + "excel file.xlsx", data_only = 'True')
plt.figure(figsize=(16,9))
axes = plt.axes()
plt.title('Drag Amplification', fontsize = 16)
plt.xlabel('Time (s)', fontsize = 14)
plt.ylabel('Cf', fontsize = 14)
d = pd.DataFrame()
n=[]
for sheets in wb.sheetnames:
if '2_1' in sheets and '2%' not in sheets and '44%' not in sheets:
name = sheets[:8]
print(name)
ws = wb[sheets]
data = ws.values
cols = next(data)[1:]
data = list(data)
idx = [r[0] for r in data]
data = (itertools.islice(r, 1, None) for r in data)
df = pd.DataFrame(data, index=idx, columns=cols)
df = df.dropna()
#x = df['x/l']
#y = df.Cf
print(df.columns)
print(df.Cf.values)
x=df['x/l'].values
plt.plot(x, df.Cf.values)
"""x = [wb[sheets].cell(row=row,column=1).value for row in range(1,2000) if wb[sheets].cell(row=row,column=1).value]
print(x)
Cf = [wb[sheets].cell(row=row,column=6).value for row in range(1,2000) if wb[sheets].cell(row=row,column=1).value]
d[name+ 'x'] = pd.DataFrame(x)
d[name + '_Cf'] = pd.Series(Cf, index=d.index)
print(name)"""
print(df)
plt.show()
I'm expecting a plot of line graphs with the values of x/l on the x access and Cf on the 'y' with a line for each of the relevant sheets in the workbook. Any insights as to why i am getting this error would be appreciated!