Compare the item-total regressions fit by the Rasch model and the interaction model, for one, several, or all items in the test.

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`x` |
A matrix of scored responses (persons in rows, items in columns) |

`items` |
The items to plot (column numbers). If NULL, all items will be plotted |

`showData` |
If TRUE, the observed proportion correct at each sum score will be shown on the plot. Default is FALSE (show only the regressions) |

`shade` |
The part of the probability mass for the sum scores to shade out,
shown as percentage. Default is 10: shade the most extreme 10
Ignored when |

`ncol` |
When plotting multiple items, the user can suggest the number of columns in the matrix of plots (up to 3, will be adjusted if necessary) |

`...` |
Any additional plotting parameters |

Ivailo Partchev, using theory and C code by Gunter Maris

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