Whole time column in csv file convert into UTC (epoch) using python
One of the easiest ways to solve the problem of plotting your time-series graph is by using pandas. You can use this code

data = pd.read_csv('YOUR_FILENAME_HERE.csv')
data['datetime'] = data['date'] + ' ' + data['time']
data['datetime'] = pd.to_datetime(data['datetime'])
data.index = data['date_time']
data.drop(['date', 'time', 'datatime'], inplace = True, axis = 1)
data['x1'].plot()