Here is a Python script that accepts file names as command line arguments and plots the data from each file on the same graph, with a pair of x and y values separated by a space on each line:

```python
import matplotlib.pyplot as plt
import sys

def plot_data(filename):
  """Plots the data from the given text file on a graph."""

  x = []
  y = []

  with open(filename, "r") as f:
    for line in f:
      x_value, y_value = line.split()
      x.append(float(x_value))
      y.append(float(y_value))

  plt.plot(x, y, label=filename)

if __name__ == "__main__":
  """Plots the data from all of the given text files on the same graph."""

  for filename in sys.argv[1:]:
    plot_data(filename)

  plt.xlabel("X")
  plt.ylabel("Y")
  plt.legend()
  plt.show()
```

To use the script, simply pass the file names of the text files containing the data that you want to plot as command line arguments. For example, to plot the data from the files `data1.txt` and `data2.txt`, you would run the following command:

```
python plot_data.py data1.txt data2.txt
```

This will produce a graph with two lines, one for each data file. The x-axis label will be "X" and the y-axis label will be "Y". There will also be a legend in the top right corner of the graph, which will identify each line.
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