Please help me to produce smoothed contour plots.

I have dependent data generated at regular intervals of two independent
variables and would like to produce smoothed contour plots - I cannot get
interp (alima) to produce cubic interpolations of the data, only linear
ones. I'm interested in smoothing as the data generation process is
stochastic and produces small variations which I'd rather 'smooth' away. I
cannot spot what is different about my data from the examples in (akima)
which prevents this working.

here is an example of my data:
> x
   0  50 100 150 200 250 300 350 400 450 500   0  50 100 150 200 250 300 350
400
 [21] 450 500   0  50 100 150 200 250 300 350 400 450 500   0  50 100 150
200 250 300
 [41] 350 400 450 500   0  50 100 150 200 250 300 350 400 450 500   0  50
100 150 200
 [61] 250 300 350 400 450 500   0  50 100 150 200 250 300 350 400 450 500
0  50 100
 [81] 150 200 250 300 350 400 450 500   0  50 100 150 200 250 300 350 400
450 500   0
[101]  50 100 150 200 250 300 350 400 450 500   0  50 100 150 200 250 300
350 400 450
[121] 500
> y
  [1] 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.05 0.05
0.05 0.05
 [17] 0.05 0.05 0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
0.10 0.10
 [33] 0.10 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.20 0.20
0.20 0.20
 [49] 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.25 0.25
 [65] 0.25 0.25 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.35
0.35 0.35
 [81] 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.40 0.40 0.40 0.40 0.40 0.40
0.40 0.40
 [97] 0.40 0.40 0.40 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45
0.50 0.50
[113] 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50
> zz
  [1] 105136  98444  95021  79375  72532  64392  57819  44413  40610  26083
18077
 [12]  96188  87863  80205  69503  61000  50387  44306  33432  22958  17575
10892
 [23]  88402  75263  65109  63717  50998  39837  28576  25960  17531
8510   5733
 [34]  70806  59245  53970  46113  38455  30492  23836  17870  10954
5218   2266
 [45]  68272  58610  47854  34834  32363  24238  19236  11789   6206
2515   2141
 [56]  58448  47706  43001  39592  22840  18264  11564   6292   2915
2228   2090
 [67]  55817  47680  38460  31693  21331  13161   8050   3162   2302
2120   1990
 [78]  54078  43720  32828  22618  17039   8524   4846   2392   2154
2068   2008
 [89]  47459  39734  29525  21325  10602   6406   3525   2207   2062
2014   1952
[100]  47630  34965  28452  18966   8386   5726   2351   2145   2052
1999   1960
[111]  42173  34339  27449  16743   9643   2866   2208   2096   2018
1937   1886

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