How to avoid initial data changing when plotting additional data in plot - matplotlib

Question

I want to plot two data series in one plot, but when plotting both data series, one of the series are changing. Matplotlib draws lines between the wrong data.
Firsty_values and secondy_values are lists of timestamps sorted and stretching one 24h interval.
Firstx_values and secondx_values are values in the range 18-21.
The first plot shows the two series together while the last plot shows one of the series alone.
#Firsty_values and secondy_values looks like this:
#['2019-05-04 00:00:03',
# '2019-05-04 00:02:03',
# ...
# '2019-05-04 23:56:03',
# '2019-05-04 23:58:02']
#Firstx_values and secondx_values looks like this:
#[18.32,18.34 ..... 19.32,19.31]
plt.plot(firsty_values,firstx_values,'b')
plt.plot(secondy_values, secondx_values, 'g')
plt.ylabel('Temperature [C]')
plt.xlabel('Time')
plt.legend(['SA1_563_04_RT601A', 'SA1_563_04_RT601B'])
plt.xticks([100,604,1053]) #length more than 1053
plt.show()
#plt.plot(firsty_values,firstx_values,'b')
plt.plot(secondy_values, secondx_values, 'g')
plt.ylabel('Temperature [C]')
plt.xlabel('Time')
plt.legend(['SA1_563_04_RT601A', 'SA1_563_04_RT601B'])
plt.xticks([100,604,1053]) #length less than 1053
plt.show()
Output:
Output with both data series :
Output with one data series :
First plot draw lines between data points that does not lie next to each other. The problem seems to be that some of the data points from the second series are put out of order after the points from the first series. This is reflected by the "xticks" showing three lables when ploting both and two lables when ploting one series.