Why is spline doing weird stuff with my data?

Hi. I'm using spline to connect some data and then access the y points of the spline for given x values with ppval. I'm doing that for a bunch of data sets but this particular one just does not want to work. I've checked the data and everything seems alright, it's just a simple logarithmic scaled curve but when I use spline it completely ignores the y data and only gives me a very small straight line.

The first image is what the data is supposed to look like and the bottom one is what spline does (plotting the ppvals of the spline against the X data below). If you could help me solve this problem I'd be very grateful.

Here is the X data:

119645.85786265088

146646.66450412222

202133.69044903992

278604.3016284374

352333.87727521197

459740.8056612833

618961.6849525509

929793.9239081189

1353657.7552896568

2017533.4544660104

3407941.7927995715

5039463.074022855

7569590.718466251

12885283.504396986

17897177.517469957

25647779.645364683

42977437.27455605

72013489.07293034

108149289.00328541

159892742.05166098

225546989.37233916

285178383.4446951

389881674.6691441

524761195.13921815

763692546.5841718

1027788923.9640564

1361818994.575842

1920731945.0032642

2708977699.2498612

4067261756.9724545

6202528142.7401905

9758910221.36459

14881929444.601082

23415342901.2511

35708132890.13138

51959273484.34699

78007014748.32399

Here is the Y data:

68.61538461538461

66.97435897435898

64.3076923076923

61.84615384615385

60.001

57.94871794871794

55.69230769230769

52.71794871794873

50.05128205128206

47.282051282051285

43.794871794871796

41.33333333333334

38.769230769230774

35.794871794871796

33.948717948717956

32.10256410256413

29.641025641025664

27.3846153846154

25.743589743589766

24.307692307692335

23.2820512820513

22.46153846153848

21.64102564102565

20.820512820512846

20.10256410256413

19.79487179487181

19.282051282051313

18.974358974359006

18.769230769230802

18.461538461538495

18.37

18.360974358974391

18.350974358974394

18.25641025641029

18.18

18.16

18.15384615384619

Answer 1

[Stephen23]

I don't see any "weird stuff":

X = [119645.85786265088;146646.66450412222;202133.69044903992;278604.3016284374;352333.87727521197;459740.8056612833;618961.6849525509;929793.9239081189;1353657.7552896568;2017533.4544660104;3407941.7927995715;5039463.074022855;7569590.718466251;12885283.504396986;17897177.517469957;25647779.645364683;42977437.27455605;72013489.07293034;108149289.00328541;159892742.05166098;225546989.37233916;285178383.4446951;389881674.6691441;524761195.13921815;763692546.5841718;1027788923.9640564;1361818994.575842;1920731945.0032642;2708977699.2498612;4067261756.9724545;6202528142.7401905;9758910221.36459;14881929444.601082;23415342901.2511;35708132890.13138;51959273484.34699;78007014748.32399];
Y = [68.61538461538461;66.97435897435898;64.3076923076923;61.84615384615385;60.001;57.94871794871794;55.69230769230769;52.71794871794873;50.05128205128206;47.282051282051285;43.794871794871796;41.33333333333334;38.769230769230774;35.794871794871796;33.948717948717956;32.10256410256413;29.641025641025664;27.3846153846154;25.743589743589766;24.307692307692335;23.2820512820513;22.46153846153848;21.64102564102565;20.820512820512846;20.10256410256413;19.79487179487181;19.282051282051313;18.974358974359006;18.769230769230802;18.461538461538495;18.37;18.360974358974391;18.350974358974394;18.25641025641029;18.18;18.16;18.15384615384619];
pp = spline(X,Y);
xq = logspace(5.5,10.5,11);
yq = ppval(pp,xq);
semilogx(X,Y,'r-',xq,yq,'b*')

[Radu Andrei Matei]

Okay, thank you, I'll look into it.