To add the option to save the plot figure with a correct title and legend, you can modify the script as follows:

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

def plot_data(filename, title, save=False):
  """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 save:
    plt.title(title)
    plt.xlabel("X")
    plt.ylabel("Y")
    plt.legend()
    plt.savefig(filename + ".png")

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, filename, save=True)

  plt.show()
```

This script now takes an additional argument, `save`, which is a Boolean value. If `save` is set to `True`, the plot figure will be saved to a file with the same name as the input file, but with a `.png` extension. The plot figure will also have a title and legend, with the title being the name of the input file.

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, followed by the `--save` flag. For example, to plot the data from the files data1.txt and data2.txt and save the plot figures, you would run the following command:

python plot_data.py data1.txt data2.txt --save

This will produce two plot figures, one for each data file, which will be saved as `data1.png` and `data2.png`. Each plot figure will have a title and legend, with the title being the name of the input file.
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