How do I raise the XY grid plane to Z=0 for a Matplotlib 3D plot?

The default 3D grid in matplotlib appears below z=0. This makes my reference lines (red) and histograms appear to be slightly off or hovering above the grid. I want them to appear to be attached to the grid.

import matplotlib.pyplot as plt
import numpy as np
from pathlib import Path
from matplotlib import cm
from scipy.stats import norm

open_file = Path('my_data_file.csv')
savefn   = Path('my_save_file.png')
nbins = 8
lsl   = 1.03
usl   = 1.05
data = np.zeros((30,8))


with open(open_file) as file:
    for x,line in enumerate(file):
        items = line.split(',')
        for y,item in enumerate(items):
            data[x,y] = float(item)           


fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')

for z in range(data.shape[1]):#loop through the data
    ys = data[:,z]
    hist, bins = np.histogram(ys, bins=nbins)
    mu, std = norm.fit(ys) #get the mean and std
    xs = (bins[:-1] + bins[1:])/2
    x = np.linspace(min(xs), max(xs), 3*len(xs))#upsample the bins
    p = norm.pdf(x,mu,std)#fit the data to a normal pdf
    p = 3*p/sum(p) #normalize fits
    y = np.ones(len(p))*(z+1)
    ax.bar(xs, hist/sum(hist), zs=z+1, zdir='y', color=cm.Set1(z), ec='k', alpha=0.5,width=(xs[1]-xs[0]))#plot the histogram
    ax.plot(x,y,p,color=cm.Set1(z))#plot the fit


ax.plot((1.03,1.03),(1,8),0,color='r')#plot the lower spec limit
ax.plot([1.05,1.05],[1,8],0,color='r')#plot the upper spec limit

ax.set_xlabel('X')
ax.set_ylabel('Y')
ax.set_zlabel('Z')
plt.xlim(lsl-(lsl/1000),usl+(lsl/1000))
plt.ylim(1,8)
ax.set_zlim([0,0.4])

plt.show()
fig.savefig(fname= savefn,format="png",dpi=600)