Visualize a Difference Pyramid

Jelly, 16 bytes

IA$ṖпUṚz”@ṚGḟ”@

Try it online!

Background

Generating the differences is quite straightforward. For input

[1, 2, 3, 4, 5, 6, 7, 8, 9]

IA$Ṗп (absolute value of increments while there is more than one element) yields the following ragged 2D array.

[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 1, 1, 1, 1, 1, 1, 1]
[0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0]
[0, 0, 0, 0]
[0, 0, 0]
[0, 0]
[0]

U reverses the order of the columns and Ṛ the order of the rows, yielding the following.

[0]
[0, 0]
[0, 0, 0]
[0, 0, 0, 0]
[0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0]
[1, 1, 1, 1, 1, 1, 1, 1]
[9, 8, 7, 6, 5, 4, 3, 2, 1]

Now, we transpose rows and columns with z”@, which pads all rows to the same length before transposing. The result is the following.

[0, 0, 0, 0, 0, 0, 0, 1, 9]
[@, 0, 0, 0, 0, 0, 0, 1, 8]
[@, @, 0, 0, 0, 0, 0, 1, 7]
[@, @, @, 0, 0, 0, 0, 1, 6]
[@, @, @, @, 0, 0, 0, 1, 5]
[@, @, @, @, @, 0, 0, 1, 4]
[@, @, @, @, @, @, 0, 1, 3]
[@, @, @, @, @, @, @, 1, 2]
[@, @, @, @, @, @, @, @, 1]

Reversing the rows gives

[@, @, @, @, @, @, @, @, 1]
[@, @, @, @, @, @, @, 1, 2]
[@, @, @, @, @, @, 0, 1, 3]
[@, @, @, @, @, 0, 0, 1, 4]
[@, @, @, @, 0, 0, 0, 1, 5]
[@, @, @, 0, 0, 0, 0, 1, 6]
[@, @, 0, 0, 0, 0, 0, 1, 7]
[@, 0, 0, 0, 0, 0, 0, 1, 8]
[0, 0, 0, 0, 0, 0, 0, 1, 9]

To turn the 2D array into a whitespace-separated string, we use the built-in grid atom (G). It joins columns by spaces and rows by linefeeds, yielding the following result.

@ @ @ @ @ @ @ @ 1
@ @ @ @ @ @ @ 1 2
@ @ @ @ @ @ 0 1 3
@ @ @ @ @ 0 0 1 4
@ @ @ @ 0 0 0 1 5
@ @ @ 0 0 0 0 1 6
@ @ 0 0 0 0 0 1 7
@ 0 0 0 0 0 0 1 8
0 0 0 0 0 0 0 1 9

All that's left to do is removing the padding character with ḟ”@, yielding

        1
       1 2
      0 1 3
     0 0 1 4
    0 0 0 1 5
   0 0 0 0 1 6
  0 0 0 0 0 1 7
 0 0 0 0 0 0 1 8
0 0 0 0 0 0 0 1 9

which is the desired output.

J, 46 42 39 36 bytes

(#\-#)|."_1#{.[:":@|./.<@#2&(|@-/\)]

A straight-forward implementation. Generates a table of successive differences between each pair and repeats it a number of times equal to the length of the input array. Then traverses each diagonal of that table to form the rows of the triangle. The rest is formatting the rows into a pyramid.

Usage

   f =: (#\-#)|."_1#{.[:":@|./.<@#2&(|@-/\)]
   f 2 5 4 9 3 4
     2     
    3 5    
   2 1 4   
  2 4 5 9  
 1 3 1 6 3 
0 1 4 5 1 4
   f 4 7
 4 
3 7
   f 8 7 3 2 4 5 9 1 6
        8        
       1 7       
      3 4 3      
     0 3 1 2     
    2 2 1 2 4    
   0 2 0 1 1 5   
  0 0 2 2 3 4 9  
 1 1 1 1 1 4 8 1 
0 1 0 1 0 1 3 5 6

Explanation

(#\-#)|."_1#{.[:":@|./.<@#2&(|@-/\)]  Input: list A
                         #            Get len(A)
                       <@             Box it
                                   ]  Get A
                          2&(     )   Repeat len(A) times on A initially
                          2      \      Get each iverlapping sublist of size 2
                                /       Reduce it using
                             |@-          The absolute value of the difference

                                      This will form a table where each row contains the
                                      successive differences
              [:     /.               Operate on the diagonals of that table
                   |.                 Reverse each diagonal
                ":@                   Format each into a string
           #                          Get len(A)
            {.                        Take that many from the strings of diagonals
 #\                                   Get the length of each prefix of A
                                      Makes the range [1, 2, ..., len(A)]
    #                                 Get len(A)
   -                                  Subtract the len(A) from each in the prefix range
                                      Makes [-len(A)+1, ..., -1, 0]
      |."_1                           Rotate each string left using each value
                                      A negative rotate left = rotate right
                                      Output the pyramid

CJam, 29 bytes

q~{_2ew::-:z}h]W%zzeeSff*W%N*

Try it online! (The first and last line enable a linefeed-separated test suite.)

This uses the triangle-rotation and layouting code from this answer and the triangle itself is generated with the same principle as in this answer.